diff --git a/Project.toml b/Project.toml
index dd4a78e..ec39530 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "SMLMData"
 uuid = "5488f106-40b8-4660-84c5-84a168990d1b"
 authors = ["klidke@unm.edu"]
-version = "0.5.1"
+version = "0.6.0"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"
diff --git a/api_overview.md b/api_overview.md
index 2e4b832..74fa60e 100644
--- a/api_overview.md
+++ b/api_overview.md
@@ -87,6 +87,7 @@ mutable struct Emitter2DFit{T} <: AbstractEmitter
     bg::T          # fitted background in photons/pixel
     σ_x::T         # uncertainty in x position in microns
     σ_y::T         # uncertainty in y position in microns
+    σ_xy::T        # covariance between x and y (microns², 0 = axis-aligned)
     σ_photons::T   # uncertainty in photon count
     σ_bg::T        # uncertainty in background level
     frame::Int     # frame number in acquisition sequence
@@ -105,6 +106,9 @@ mutable struct Emitter3DFit{T} <: AbstractEmitter
     σ_x::T         # uncertainty in x position in microns
     σ_y::T         # uncertainty in y position in microns
     σ_z::T         # uncertainty in z position in microns
+    σ_xy::T        # covariance between x and y (microns², 0 = uncorrelated)
+    σ_xz::T        # covariance between x and z (microns², 0 = uncorrelated)
+    σ_yz::T        # covariance between y and z (microns², 0 = uncorrelated)
     σ_photons::T   # uncertainty in photon count
     σ_bg::T        # uncertainty in background level
     frame::Int     # frame number in acquisition sequence
@@ -138,6 +142,7 @@ emitter_2d_fit = Emitter2DFit{Float64}(
     1000.0, 10.0,    # photons detected, background photons/pixel
     0.01, 0.01,      # σ_x, σ_y: position uncertainties in microns
     50.0, 2.0;       # σ_photons, σ_bg: photon count uncertainties
+    σ_xy=0.005,      # covariance (optional, default=0 for axis-aligned uncertainty)
     frame=5,         # frame number in acquisition (1-based, default=1)
     dataset=1,       # dataset identifier for multi-acquisition experiments
     track_id=2,      # tracking ID for linked localizations (default=0 = unlinked)
diff --git a/docs/src/index.md b/docs/src/index.md
index 17925fb..a01e830 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -71,6 +71,7 @@ mutable struct Emitter2DFit{T} <: AbstractEmitter
     bg::T          # fitted background in photons/pixel
     σ_x::T         # uncertainty in x position in microns
     σ_y::T         # uncertainty in y position in microns
+    σ_xy::T        # covariance between x and y (microns², 0 = axis-aligned)
     σ_photons::T   # uncertainty in photon count
     σ_bg::T        # uncertainty in background level
     frame::Int     # frame number in acquisition sequence
@@ -88,6 +89,9 @@ mutable struct Emitter3DFit{T} <: AbstractEmitter
     σ_x::T         # uncertainty in x position in microns
     σ_y::T         # uncertainty in y position in microns
     σ_z::T         # uncertainty in z position in microns
+    σ_xy::T        # covariance between x and y (microns², 0 = uncorrelated)
+    σ_xz::T        # covariance between x and z (microns², 0 = uncorrelated)
+    σ_yz::T        # covariance between y and z (microns², 0 = uncorrelated)
     σ_photons::T   # uncertainty in photon count
     σ_bg::T        # uncertainty in background level
     frame::Int     # frame number in acquisition sequence
@@ -118,6 +122,7 @@ emitter_2d_fit = Emitter2DFit{Float64}(
     1000.0, 10.0,    # photons, background
     0.01, 0.01,      # σ_x, σ_y (uncertainties in μm)
     50.0, 2.0;       # σ_photons, σ_bg (uncertainties)
+    σ_xy=0.005,      # covariance (optional, 0 = axis-aligned)
     frame=5,         # frame number
     dataset=1,       # dataset identifier
     track_id=2,      # tracking identifier (0 = unlinked)
@@ -130,6 +135,7 @@ emitter_3d_fit = Emitter3DFit{Float64}(
     1000.0, 10.0,      # photons, background
     0.01, 0.01, 0.02,  # σ_x, σ_y, σ_z (uncertainties in μm)
     50.0, 2.0;         # σ_photons, σ_bg (uncertainties)
+    σ_xy=0.005, σ_xz=0.002, σ_yz=0.003,  # covariances (optional, 0 = uncorrelated)
     frame=5,           # frame number
     dataset=1,         # dataset identifier
     track_id=2,        # tracking identifier (0 = unlinked)
diff --git a/src/types/emitters.jl b/src/types/emitters.jl
index c55acbc..5a37e02 100644
--- a/src/types/emitters.jl
+++ b/src/types/emitters.jl
@@ -52,6 +52,7 @@ Represents fitted 2D localization results with uncertainties and temporal/tracki
 - `bg::T`: fitted background in photons/pixel
 - `σ_x::T`: uncertainty in x position in microns
 - `σ_y::T`: uncertainty in y position in microns
+- `σ_xy::T`: covariance between x and y uncertainties (microns², 0 = axis-aligned)
 - `σ_photons::T`: uncertainty in photon count
 - `σ_bg::T`: uncertainty in background in photons/pixel
 - `frame::Int`: frame number in acquisition sequence
@@ -66,6 +67,7 @@ mutable struct Emitter2DFit{T} <: AbstractEmitter
     bg::T
     σ_x::T
     σ_y::T
+    σ_xy::T
     σ_photons::T
     σ_bg::T
     frame::Int
@@ -88,6 +90,9 @@ Represents fitted 3D localization results with uncertainties and temporal/tracki
 - `σ_x::T`: uncertainty in x position in microns
 - `σ_y::T`: uncertainty in y position in microns
 - `σ_z::T`: uncertainty in z position in microns
+- `σ_xy::T`: covariance between x and y (microns², 0 = uncorrelated)
+- `σ_xz::T`: covariance between x and z (microns², 0 = uncorrelated)
+- `σ_yz::T`: covariance between y and z (microns², 0 = uncorrelated)
 - `σ_photons::T`: uncertainty in photon count
 - `σ_bg::T`: uncertainty in background in photons/pixel
 - `frame::Int`: frame number in acquisition sequence
@@ -104,6 +109,9 @@ mutable struct Emitter3DFit{T} <: AbstractEmitter
     σ_x::T
     σ_y::T
     σ_z::T
+    σ_xy::T
+    σ_xz::T
+    σ_yz::T
     σ_photons::T
     σ_bg::T
     frame::Int
@@ -115,7 +123,7 @@ end
 
 """
     Emitter2DFit{T}(x, y, photons, bg, σ_x, σ_y, σ_photons, σ_bg;
-                    frame=0, dataset=1, track_id=0, id=0) where T
+                    σ_xy=zero(T), frame=1, dataset=1, track_id=0, id=0) where T
 
 Convenience constructor for 2D localization fit results with optional identification parameters.
 
@@ -131,6 +139,7 @@ Convenience constructor for 2D localization fit results with optional identifica
 - `σ_bg::T`: uncertainty in background level
 
 ## Optional Keywords
+- `σ_xy::T=0`: covariance between x and y uncertainties (microns², 0 = axis-aligned)
 - `frame::Int=1`: frame number in acquisition sequence
 - `dataset::Int=1`: identifier for specific acquisition/dataset
 - `track_id::Int=0`: identifier for linking localizations across frames
@@ -146,23 +155,24 @@ emitter = Emitter2DFit{Float64}(
     50.0, 2.0        # σ_photons, σ_bg
 )
 
-# Create emitter with specific frame and dataset
+# Create emitter with covariance for rotated uncertainty ellipse
 emitter = Emitter2DFit{Float64}(
     1.0, 2.0, 1000.0, 10.0, 0.01, 0.01, 50.0, 2.0;
-    frame=5, dataset=2
+    σ_xy=0.005, frame=5, dataset=2
 )
 ```
 """
-function Emitter2DFit{T}(x::T, y::T, photons::T, bg::T, 
+function Emitter2DFit{T}(x::T, y::T, photons::T, bg::T,
                         σ_x::T, σ_y::T, σ_photons::T, σ_bg::T;
-                        frame::Int=1, dataset::Int=1, track_id::Int=0, id::Int=0) where T
-    Emitter2DFit{T}(x, y, photons, bg, σ_x, σ_y, σ_photons, σ_bg, 
+                        σ_xy::T=zero(T), frame::Int=1, dataset::Int=1, track_id::Int=0, id::Int=0) where T
+    Emitter2DFit{T}(x, y, photons, bg, σ_x, σ_y, σ_xy, σ_photons, σ_bg,
                     frame, dataset, track_id, id)
 end
 
 """
     Emitter3DFit{T}(x, y, z, photons, bg, σ_x, σ_y, σ_z, σ_photons, σ_bg;
-                    frame=0, dataset=1, track_id=0, id=0) where T
+                    σ_xy=zero(T), σ_xz=zero(T), σ_yz=zero(T),
+                    frame=1, dataset=1, track_id=0, id=0) where T
 
 Convenience constructor for 3D localization fit results with optional identification parameters.
 
@@ -180,6 +190,9 @@ Convenience constructor for 3D localization fit results with optional identifica
 - `σ_bg::T`: uncertainty in background level
 
 ## Optional Keywords
+- `σ_xy::T=0`: covariance between x and y (microns², 0 = uncorrelated)
+- `σ_xz::T=0`: covariance between x and z (microns², 0 = uncorrelated)
+- `σ_yz::T=0`: covariance between y and z (microns², 0 = uncorrelated)
 - `frame::Int=1`: frame number in acquisition sequence
 - `dataset::Int=1`: identifier for specific acquisition/dataset
 - `track_id::Int=0`: identifier for linking localizations across frames
@@ -195,17 +208,18 @@ emitter = Emitter3DFit{Float64}(
     50.0, 2.0        # σ_photons, σ_bg
 )
 
-# Create emitter with specific frame and tracking
+# Create emitter with full 3D covariance
 emitter = Emitter3DFit{Float64}(
     1.0, 2.0, -0.5, 1000.0, 10.0, 0.01, 0.01, 0.02, 50.0, 2.0;
-    frame=5, track_id=1
+    σ_xy=0.005, σ_xz=0.002, σ_yz=0.003, frame=5, track_id=1
 )
 ```
 """
-function Emitter3DFit{T}(x::T, y::T, z::T, photons::T, bg::T, 
+function Emitter3DFit{T}(x::T, y::T, z::T, photons::T, bg::T,
                         σ_x::T, σ_y::T, σ_z::T, σ_photons::T, σ_bg::T;
+                        σ_xy::T=zero(T), σ_xz::T=zero(T), σ_yz::T=zero(T),
                         frame::Int=1, dataset::Int=1, track_id::Int=0, id::Int=0) where T
-    Emitter3DFit{T}(x, y, z, photons, bg, σ_x, σ_y, σ_z, σ_photons, σ_bg,
+    Emitter3DFit{T}(x, y, z, photons, bg, σ_x, σ_y, σ_z, σ_xy, σ_xz, σ_yz, σ_photons, σ_bg,
                     frame, dataset, track_id, id)
 end
 
@@ -263,6 +277,7 @@ function Base.show(io::IO, ::MIME"text/plain", e::Emitter2DFit{T}) where T
     println(io, "  Uncertainties:")
     println(io, "    σ_x: $(e.σ_x) μm")
     println(io, "    σ_y: $(e.σ_y) μm")
+    e.σ_xy != 0 && println(io, "    σ_xy: $(e.σ_xy) μm²")
     println(io, "    σ_photons: $(e.σ_photons)")
     println(io, "    σ_bg: $(e.σ_bg)")
     println(io, "  Frame: $(e.frame)")
@@ -289,6 +304,13 @@ function Base.show(io::IO, ::MIME"text/plain", e::Emitter3DFit{T}) where T
     println(io, "    σ_x: $(e.σ_x) μm")
     println(io, "    σ_y: $(e.σ_y) μm")
     println(io, "    σ_z: $(e.σ_z) μm")
+    has_cov = e.σ_xy != 0 || e.σ_xz != 0 || e.σ_yz != 0
+    if has_cov
+        println(io, "  Covariances:")
+        e.σ_xy != 0 && println(io, "    σ_xy: $(e.σ_xy) μm²")
+        e.σ_xz != 0 && println(io, "    σ_xz: $(e.σ_xz) μm²")
+        e.σ_yz != 0 && println(io, "    σ_yz: $(e.σ_yz) μm²")
+    end
     println(io, "    σ_photons: $(e.σ_photons)")
     println(io, "    σ_bg: $(e.σ_bg)")
     println(io, "  Frame: $(e.frame)")
diff --git a/test/test_emitters.jl b/test/test_emitters.jl
index 68a9727..afe081b 100644
--- a/test/test_emitters.jl
+++ b/test/test_emitters.jl
@@ -27,15 +27,17 @@ end
             1.0, 2.0,        # x, y
             1000.0, 10.0,    # photons, bg
             0.01, 0.01,      # σ_x, σ_y
+            0.005,           # σ_xy (covariance)
             50.0, 2.0,       # σ_photons, σ_bg
             1, 1, 0, 1       # frame, dataset, track_id, id
         )
         @test e2df.x == 1.0
         @test e2df.σ_x == 0.01
+        @test e2df.σ_xy == 0.005
         @test e2df.frame == 1
         @test e2df.track_id == 0
-        
-        # Test convenience constructor
+
+        # Test convenience constructor (σ_xy defaults to 0)
         e2df_simple = Emitter2DFit{Float64}(
             1.0, 2.0, 1000.0, 10.0, 0.01, 0.01, 50.0, 2.0
         )
@@ -43,27 +45,52 @@ end
         @test e2df_simple.dataset == 1  # default value
         @test e2df_simple.track_id == 0  # default value
         @test e2df_simple.id == 0  # default value
+        @test e2df_simple.σ_xy == 0.0  # default covariance
+
+        # Test convenience constructor with σ_xy
+        e2df_cov = Emitter2DFit{Float64}(
+            1.0, 2.0, 1000.0, 10.0, 0.01, 0.01, 50.0, 2.0;
+            σ_xy=0.003
+        )
+        @test e2df_cov.σ_xy == 0.003
     end
 
     @testset "3D Fit" begin
         # Test full constructor
         e3df = Emitter3DFit{Float64}(
-            1.0, 2.0, 3.0,    # x, y, z
-            1000.0, 10.0,     # photons, bg
-            0.01, 0.01, 0.02, # σ_x, σ_y, σ_z
-            50.0, 2.0,        # σ_photons, σ_bg
-            1, 1, 0, 1        # frame, dataset, track_id, id
+            1.0, 2.0, 3.0,       # x, y, z
+            1000.0, 10.0,        # photons, bg
+            0.01, 0.01, 0.02,    # σ_x, σ_y, σ_z
+            0.005, 0.002, 0.003, # σ_xy, σ_xz, σ_yz (covariances)
+            50.0, 2.0,           # σ_photons, σ_bg
+            1, 1, 0, 1           # frame, dataset, track_id, id
         )
         @test e3df.z == 3.0
         @test e3df.σ_z == 0.02
-        
-        # Test convenience constructor
+        @test e3df.σ_xy == 0.005
+        @test e3df.σ_xz == 0.002
+        @test e3df.σ_yz == 0.003
+
+        # Test convenience constructor (covariances default to 0)
         e3df_simple = Emitter3DFit{Float64}(
-            1.0, 2.0, 3.0, 1000.0, 10.0, 
+            1.0, 2.0, 3.0, 1000.0, 10.0,
             0.01, 0.01, 0.02, 50.0, 2.0
         )
         @test e3df_simple.frame == 1
         @test e3df_simple.dataset == 1
+        @test e3df_simple.σ_xy == 0.0
+        @test e3df_simple.σ_xz == 0.0
+        @test e3df_simple.σ_yz == 0.0
+
+        # Test convenience constructor with full 3D covariance
+        e3df_cov = Emitter3DFit{Float64}(
+            1.0, 2.0, 3.0, 1000.0, 10.0,
+            0.01, 0.01, 0.02, 50.0, 2.0;
+            σ_xy=0.003, σ_xz=0.001, σ_yz=0.002
+        )
+        @test e3df_cov.σ_xy == 0.003
+        @test e3df_cov.σ_xz == 0.001
+        @test e3df_cov.σ_yz == 0.002
     end
 end
 
@@ -71,11 +98,12 @@ end
     # Test that operations maintain type stability
     e2d = Emitter2D{Float64}(1.0, 2.0, 1000.0)
     @test typeof(e2d.x) === Float64
-    
+
     e2df = Emitter2DFit{Float32}(
         1.0f0, 2.0f0, 1000.0f0, 10.0f0,
         0.01f0, 0.01f0, 50.0f0, 2.0f0
     )
     @test typeof(e2df.x) === Float32
     @test typeof(e2df.σ_x) === Float32
+    @test typeof(e2df.σ_xy) === Float32
 end