Patterns are well understood methodology for object-oriented software architecture, especially for business applications. Scientific programmers have generally avoided object-oriented approaches because of their heavy computational overhead, but the benefits of using patterns for scientific problems can outweigh their costs. This article introduces the concept of object oriented software patterns and discusses how they can be applied to scientific software problems. After a brief explanation of what patterns are and why they can be relevant to scientific software, the author explores the application of patterns to dynamic-systems simulation, such as molecular dynamics, and identifies four design patterns that emerge in modeling such systems. To illustrate how to reuse a general pattern for a specific problem, he applies one of the dynamic simulation patterns to the different problem of hydrodynamic chemistry tracers.