  Cosmological parameter estimation is entering a new era. Large collaborations
need to coordinate high-stakes analyses using multiple methods; furthermore
such analyses have grown in complexity due to sophisticated models of cosmology
and systematic uncertainties. In this paper we argue that modularity is the key
to addressing these challenges: calculations should be broken up into
interchangeable modular units with inputs and outputs clearly defined. We
present a new framework for cosmological parameter estimation, CosmoSIS,
designed to connect together, share, and advance development of inference tools
across the community. We describe the modules already available in CosmoSIS,
including CAMB, Planck, cosmic shear calculations, and a suite of samplers. We
illustrate it using demonstration code that you can run out-of-the-box with the
installer available at http://bitbucket.org/joezuntz/cosmosis
