diff --git a/Package.swift b/Package.swift
index 9022b6f..193727b 100644
--- a/Package.swift
+++ b/Package.swift
@@ -95,6 +95,15 @@ let package = Package(
             resources: [.process("Localized.yml")],
             plugins: [.plugin(name: "GenerateLocalized", package: "localized")]
         ),
+
+        .testTarget(
+            name: "LuminateTests",
+            dependencies: [
+                "CAspectContainer",
+                .product(name: "CAdw", package: "adwaita-swift"),
+                .product(name: "Adwaita", package: "adwaita-swift"),
+            ]
+        ),
     ],
     swiftLanguageModes: [.v5]
 )
diff --git a/Tests/LuminateTests/WidgetTests.swift b/Tests/LuminateTests/WidgetTests.swift
new file mode 100644
index 0000000..180c356
--- /dev/null
+++ b/Tests/LuminateTests/WidgetTests.swift
@@ -0,0 +1,187 @@
+import Adwaita
+import CAdw
+import CAspectContainer
+import Testing
+
+// MARK: - Helpers
+
+/// Initialize GTK once per test suite.
+func ensureGTKInit() {
+    struct Once {
+        static let initialized: Bool = {
+            gtk_init()
+            return true
+        }()
+    }
+    _ = Once.initialized
+}
+
+/// Create a simple test widget that reports a fixed size.
+func makeFixedChild(width: Int, height: Int) -> UnsafeMutablePointer<GtkWidget>? {
+    let box = gtk_box_new(.GTK_ORIENTATION_VERTICAL, 0)
+    gtk_widget_set_size_request(box, Int32(width), Int32(height))
+    gtk_widget_realize(box)
+    return box
+}
+
+/// Measure a widget and return (minimum, natural).
+func measureWidget(
+    _ widget: UnsafeMutablePointer<GtkWidget>?,
+    orientation: GtkOrientation,
+    forSize: Int = -1
+) -> (minimum: Int, natural: Int) {
+    var min: Int32 = 0
+    var nat: Int32 = 0
+    gtk_widget_measure(widget, orientation, Int32(forSize), &min, &nat, nil, nil)
+    return (Int(min), Int(nat))
+}
+
+// MARK: - AspectContainer Tests
+
+@Suite("Widget Tests", .serialized)
+struct WidgetTests {
+
+    init() {
+        ensureGTKInit()
+    }
+
+    @Test("Reports height = width × ratio when for_size is known")
+    func measureWithKnownWidth() {
+        let container = aspect_container_new(2.0)!
+        let child = makeFixedChild(width: 100, height: 50)
+        gtk_widget_set_parent(child, container)
+
+        let (minH, natH) = measureWidget(
+            container, orientation: .GTK_ORIENTATION_VERTICAL, forSize: 100)
+        #expect(minH == 200, "height = 100 × 2.0 = 200")
+        #expect(natH == 200, "natural height = 200")
+    }
+
+    @Test("Reports height = child_min_w × ratio when for_size is unknown")
+    func measureFallback() {
+        let container = aspect_container_new(1.5)!
+        let child = makeFixedChild(width: 100, height: 100)
+        gtk_widget_set_parent(child, container)
+        gtk_widget_set_size_request(container, 100, -1)
+
+        let (minH, natH) = measureWidget(
+            container, orientation: .GTK_ORIENTATION_VERTICAL, forSize: -1)
+        #expect(minH == 150, "height = 100 × 1.5 = 150 (minimum)")
+        #expect(natH == 150, "natural = 150")
+    }
+
+    @Test("max_width caps reported width")
+    func maxWidthCapsWidth() {
+        let container = aspect_container_new(1.5)!
+        let child = makeFixedChild(width: 300, height: 300)
+        gtk_widget_set_parent(child, container)
+        aspect_container_set_max_width(container.opaque(), 200)
+
+        let (minW, natW) = measureWidget(
+            container, orientation: .GTK_ORIENTATION_HORIZONTAL, forSize: -1)
+        #expect(minW == 200, "capped at 200")
+        #expect(natW == 200, "capped at 200")
+    }
+
+    @Test("max_width affects height calculation")
+    func maxWidthAffectsHeight() {
+        let container = aspect_container_new(2.0)!
+        let child = makeFixedChild(width: 300, height: 150)
+        gtk_widget_set_parent(child, container)
+        aspect_container_set_max_width(container.opaque(), 100)
+
+        let (minH, natH) = measureWidget(
+            container, orientation: .GTK_ORIENTATION_VERTICAL, forSize: -1)
+        #expect(minH == 200, "height = max_width × ratio = 200")
+        #expect(natH == 200, "natural = 200")
+    }
+
+    @Test("Changing aspect ratio triggers resize")
+    func changeAspectRatio() {
+        let container = aspect_container_new(1.0)!
+        let child = makeFixedChild(width: 100, height: 100)
+        gtk_widget_set_parent(child, container)
+        aspect_container_set_aspect_ratio(container.opaque(), 0.5)
+
+        let (minH, natH) = measureWidget(
+            container, orientation: .GTK_ORIENTATION_VERTICAL, forSize: 100)
+        #expect(minH == 50, "height = 100 × 0.5 = 50")
+        #expect(natH == 50, "natural = 50")
+    }
+
+    @Test("Horizontal measure returns 0 without max_width")
+    func horizontalMeasureNoMaxWidth() {
+        let container = aspect_container_new(1.5)!
+        let child = makeFixedChild(width: 300, height: 200)
+        gtk_widget_set_parent(child, container)
+
+        let (minW, natW) = measureWidget(
+            container, orientation: .GTK_ORIENTATION_HORIZONTAL, forSize: -1)
+        #expect(minW == 0, "no max_width → reports 0")
+        #expect(natW == 0, "no max_width → reports 0")
+    }
+
+    // MARK: - AspectContainer Tests
+
+    /// Helper: create a FlowGrid with `n` children, each having a given size.
+    func makeFlowGrid(
+        minimumSize: Int = 100,
+        columnSpacing: Int = 0,
+        rowSpacing: Int = 0,
+        childSize: Int = 100,
+        count: Int = 4
+    ) -> UnsafeMutablePointer<GtkWidget>? {
+        let grid = flow_grid_new(Int32(minimumSize), Int32(columnSpacing), Int32(rowSpacing))
+        for _ in 0..<count {
+            let child = gtk_box_new(.GTK_ORIENTATION_VERTICAL, 0)
+            gtk_widget_set_size_request(child, Int32(childSize), Int32(childSize))
+            gtk_widget_set_parent(child, grid)
+        }
+        return grid
+    }
+
+    @Test("Height = tallest child height when for_size is known")
+    func measureTalliesRowHeights() {
+        let grid = makeFlowGrid(
+            minimumSize: 150, columnSpacing: 0, rowSpacing: 0, childSize: 100, count: 6)
+
+        let (minH, natH) = measureWidget(grid, orientation: .GTK_ORIENTATION_VERTICAL, forSize: 300)
+        #expect(minH == 300, "3 rows × 100 = 300")
+        #expect(natH == 300, "natural = 300")
+    }
+
+    @Test("Column spacing is accounted for in column count")
+    func columnSpacingAffectsCount() {
+        let noSpacing = makeFlowGrid(
+            minimumSize: 100, columnSpacing: 0, rowSpacing: 0, childSize: 100, count: 4)
+        let withSpacing = makeFlowGrid(
+            minimumSize: 100, columnSpacing: 50, rowSpacing: 0, childSize: 100, count: 4)
+
+        let (h1, _) = measureWidget(noSpacing, orientation: .GTK_ORIENTATION_VERTICAL, forSize: 250)
+        #expect(h1 == 200, "250 width → 2 cols × 2 rows × 100 = 200")
+
+        let (h2, _) = measureWidget(
+            withSpacing, orientation: .GTK_ORIENTATION_VERTICAL, forSize: 250)
+        #expect(h2 == 200, "250 width → 2 cols × 2 rows × 100 = 200")
+    }
+
+    @Test("Row spacing adds to total height")
+    func rowSpacingAddsHeight() {
+        let grid = makeFlowGrid(
+            minimumSize: 100, columnSpacing: 0, rowSpacing: 10, childSize: 100, count: 6)
+
+        let (minH, natH) = measureWidget(grid, orientation: .GTK_ORIENTATION_VERTICAL, forSize: 300)
+        #expect(minH == 210, "2 rows × 100 + 1 spacing × 10 = 210")
+        #expect(natH == 210, "natural = 210")
+    }
+
+    @Test("Single child fills one cell")
+    func singleChild() {
+        let grid = makeFlowGrid(minimumSize: 200, childSize: 150, count: 1)
+
+        let (minH, natH) = measureWidget(grid, orientation: .GTK_ORIENTATION_VERTICAL, forSize: 400)
+        #expect(minH == 150, "single row = child height")
+        #expect(natH == 150, "natural = 150")
+    }
+
+}