The Community Climate System Model (CCSM) is a com- puter model for simulating the Earth's climate. The CCSM is built from four individual component models for the atmosphere, ocean, land surface, and sea ice. The notion of a physical/dynamical component of the climate system translates directly to the software component structure. Software design of the CCSM is focused on the goals of modularity, extensibility, and performance portability. These goals are met at both the component level and within the individual component models. Performance portability is the ability of a code to achieve good perform- ance across a variety of computer architectures while maintaining a single source code. As a community model, the CCSM must run on a variety of machine architectures and must perform well on all these architectures for com- putationally intensive climate simulations.