Polymorphism-the ability of a client to send a message that is implemented by objects of different types without needing to know the exact type of the object that will respond to the message-is a cornerstone of object-oriented programming. It is intended as a way to facilitate the modularity and extensibility of software systems by hiding the variability in behavior behind a unified interface and allowing the client and provider to be decoupled in their evolution. The downside of polymorphism is that it scatters the implementation of the behavior over multiple classes, which could result in code that is harder to understand. In this research we study how polymorphism is used in one of the oldest and in one of the most widespread object-oriented programming languages: Smalltalk and Java. For this we study a large corpus of several hundreds of systems to answer questions about the prevalence of polymorphism in practice. We also estimate the burden polymorphism imposes on developers maintaining software systems by determining how scattered are the implementations of polymorphic methods in class hierarchies.