diff --git a/CHANGELOG.md b/CHANGELOG.md
index ba10f4b..9008f1b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,6 @@
+## Unreleased
+- Add support for merging ([#23](https://github.com/avo-hq/class_variants/pull/23))
+
 ## 1.0.0 (2024-11-13)
 - Add support for slots ([#15](https://github.com/avo-hq/class_variants/pull/15))
 - Allow passing additional classes when render ([#17](https://github.com/avo-hq/class_variants/pull/17))
diff --git a/lib/class_variants/instance.rb b/lib/class_variants/instance.rb
index 55c170e..e6e4a7b 100644
--- a/lib/class_variants/instance.rb
+++ b/lib/class_variants/instance.rb
@@ -1,20 +1,36 @@
 module ClassVariants
   class Instance
-    def initialize(**options, &block)
+    def initialize(...)
+      @bases = []
+      @variants = []
+      @defaults = {}
+
+      merge(...)
+    end
+
+    def merge(**options, &block)
       raise ArgumentError, "Use of hash config and code block is not supported" if !options.empty? && block_given?
 
-      @base = options.empty? ? {} : {default: options.fetch(:base, nil)}
-      @variants = expand_variants(options.fetch(:variants, {})) + expand_compound_variants(options.fetch(:compound_variants, []))
-      @defaults = options.fetch(:defaults, {})
+      (base = options.fetch(:base, nil)) && @bases << {class: base, slot: :default}
+      @variants += [
+        expand_variants(options.fetch(:variants, {})),
+        expand_compound_variants(options.fetch(:compound_variants, []))
+      ].inject(:+)
+      @defaults.merge!(options.fetch(:defaults, {}))
 
       instance_eval(&block) if block_given?
+
+      self
     end
 
     def render(slot = :default, **overrides)
       classes = overrides.delete(:class)
+      result = []
 
       # Start with our default classes
-      result = [@base[slot]]
+      @bases.each do |base|
+        result << base[:class] if base[:slot] == slot
+      end
 
       # Then merge the passed in overrides on top of the defaults
       criteria = @defaults.merge(overrides)
@@ -44,10 +60,10 @@ def base(klass = nil, &block)
 
       if block_given?
         with_slots(&block).each do |slot|
-          @base[slot[:slot]] = slot[:class]
+          @bases << slot
         end
       else
-        @base[:default] = klass
+        @bases << {slot: :default, class: klass}
       end
     end
 
diff --git a/test/merge_test.rb b/test/merge_test.rb
new file mode 100644
index 0000000..7db792d
--- /dev/null
+++ b/test/merge_test.rb
@@ -0,0 +1,71 @@
+require "test_helper"
+
+class MergeTest < Minitest::Test
+  def test_hash_merge
+    cv = ClassVariants.build(
+      base: "rounded",
+      variants: {
+        color: {
+          primary: "bg-blue-500",
+          secondary: "bg-purple-500",
+          success: "bg-green-500"
+        }
+      },
+      default: {
+        color: :primary
+      }
+    )
+    cv.merge(
+      base: "border",
+      variants: {
+        color: {
+          primary: "bg-blue-700",
+          secondary: "bg-purple-700"
+        }
+      },
+      defaults: {
+        color: :secondary
+      }
+    )
+
+    assert_equal "rounded border bg-purple-500 bg-purple-700", cv.render
+  end
+
+  def test_block_merge
+    cv = ClassVariants.build do
+      base do
+        slot :root, class: "rounded"
+        slot :title, class: "font-bold"
+      end
+
+      variant variant: :outlined do
+        slot :root, class: "border-red-700"
+        slot :title, class: "text-red-700"
+      end
+
+      variant variant: :filled do
+        slot :root, class: "bg-red-100"
+        slot :title, class: "text-red-900"
+      end
+
+      defaults variant: :outlined
+    end
+
+    cv.merge do
+      base do
+        slot :root, class: "mb-4"
+        slot :title, class: "mb-1"
+      end
+
+      variant variant: :filled do
+        slot :root, class: "dark:bg-red-800"
+        slot :title, class: "dark:text-red-50"
+      end
+
+      defaults variant: :filled
+    end
+
+    assert_equal "rounded mb-4 bg-red-100 dark:bg-red-800", cv.render(:root)
+    assert_equal "font-bold mb-1 text-red-900 dark:text-red-50", cv.render(:title)
+  end
+end