Abstract. Two object-oriented programming language paradigmsdynamic, prototype-based languages and multi-method languagesprovide orthogonal benefits to software engineers. These two paradigms appear to be in conflict, however, preventing engineers from realizing the benefits of both technologies in one system. This paper introduces a novel object model, prototypes with multiple dispatch (PMD), which seamlessly unifies these two approaches. We give formal semantics for PMD, and discuss implementation and experience with PMD in the dynamically typed programming language Slate. OverviewWe begin the paper by describing a motivating example that shows the limitations of current, popular object-oriented languages for capturing how method behavior depends on the interaction between objects and their state. The example shows that multi-methods can cleanly capture how behavior depends on the interaction between objects, while dynamic, prototype-based languages can cleanly capture how behavior depends on object state. Unfortunately, unifying highly dynamic, prototype-based languages with multi-methods is hard, because traditional multi-methods assume a static class hierarchy that is not present in dynamic prototype-based languages.In section 3 we describe Prototypes with Multiple Dispatch (PMD), an object model that combines the benefits of dynamic, prototype-based languages with multi-methods. PMD supports both paradigms by introducing a role concept that links a slot within an object to a dispatch position on a method, and defining a dynamic multi-method dispatch mechanism that traverses the graph of objects, methods, and roles to find the most specific method implementation for a given set of receiver objects.Section 4 defines the PMD model more precisely using operational semantics. Section 5 demonstrates the expressiveness of PMD through the standard library of Slate, a dynamically-typed language that implements the PMD object model. Section 6 describes an efficient algorithm for implementing dispatch in Slate. Section 7 describes related work, and section 8 concludes.