diff --git a/lib/prism/orm/errors.rb b/lib/prism/orm/errors.rb
new file mode 100644
index 0000000..eb78c6d
--- /dev/null
+++ b/lib/prism/orm/errors.rb
@@ -0,0 +1,43 @@
+module Prism
+  module Orm
+    class Errors
+      include Enumerable
+
+      def initialize
+        @messages = Hash.new { |hash, key| hash[key] = [] }
+      end
+
+      def add(attribute, message)
+        @messages[attribute.to_sym] << message
+      end
+
+      def [](attribute)
+        @messages[attribute.to_sym]
+      end
+
+      def key?(attribute)
+        @messages.key?(attribute.to_sym)
+      end
+
+      def clear
+        @messages.clear
+      end
+
+      def empty?
+        @messages.empty?
+      end
+
+      def any?
+        !empty?
+      end
+
+      def each(&block)
+        @messages.each(&block)
+      end
+
+      def full_messages
+        @messages.flat_map { |attribute, messages| messages.map { |message| "#{attribute} #{message}" } }
+      end
+    end
+  end
+end
diff --git a/lib/prism/orm/validatable.rb b/lib/prism/orm/validatable.rb
new file mode 100644
index 0000000..32c3325
--- /dev/null
+++ b/lib/prism/orm/validatable.rb
@@ -0,0 +1,81 @@
+module Prism
+  module Orm
+    module Validatable
+      def self.included(base)
+        base.extend(ClassMethods)
+      end
+
+      module ClassMethods
+        def validators
+          @validators ||= []
+        end
+
+        def validates(*attributes, **rules)
+          attributes.each do |attribute|
+            rules.each { |kind, config| validators << { attribute: attribute, kind: kind, config: config } }
+          end
+        end
+      end
+
+      def errors
+        @errors ||= Errors.new
+      end
+
+      def valid?
+        errors.clear
+        self.class.validators.each { |validator| run_validator(validator) }
+        errors.empty?
+      end
+
+      def invalid?
+        !valid?
+      end
+
+      private
+
+      def run_validator(validator)
+        case validator[:kind]
+        when :presence then validate_presence(validator[:attribute], validator[:config])
+        when :uniqueness then validate_uniqueness(validator[:attribute], validator[:config])
+        when :format then validate_format(validator[:attribute], validator[:config])
+        when :length then validate_length(validator[:attribute], validator[:config])
+        else raise ArgumentError, "unknown validator #{validator[:kind]}"
+        end
+      end
+
+      def validate_presence(attribute, config)
+        return unless config
+        value = self[attribute]
+        return unless value.nil? || (value.respond_to?(:empty?) && value.empty?)
+        errors.add(attribute, message_for(config, 'must be present'))
+      end
+
+      def validate_uniqueness(attribute, config)
+        return unless config
+        value = self[attribute]
+        return if value.nil?
+        conflict = self.class.where(attribute => value).any? { |record| !persisted? || record.id != id }
+        errors.add(attribute, message_for(config, 'must be unique')) if conflict
+      end
+
+      def validate_format(attribute, config)
+        pattern = config.is_a?(Hash) ? config[:with] : config
+        return unless pattern
+        value = self[attribute]
+        return if value.nil?
+        return if value.to_s.match?(pattern)
+        errors.add(attribute, message_for(config, 'is invalid'))
+      end
+
+      def validate_length(attribute, config)
+        value = self[attribute].to_s
+        errors.add(attribute, "is too short (minimum #{config[:minimum]})") if config[:minimum] && value.length < config[:minimum]
+        errors.add(attribute, "is too long (maximum #{config[:maximum]})") if config[:maximum] && value.length > config[:maximum]
+      end
+
+      def message_for(config, default)
+        config.is_a?(Hash) && config[:message] ? config[:message] : default
+      end
+    end
+  end
+end