PySB is a Python framework for building and simulating rule-based mathematical models of biochemical systems. It provides a domain-specific language (DSL) embedded in Python for defining monomers, rules, parameters, and observables, with operator overloading to express reactions concisely. The library supports multiple simulation backends (SciPy ODE, BioNetGen, StochKit, Kappa, CUDA/OpenCL SSA, cupSODA) and can export models to SBML, MATLAB, Mathematica, BNGL, JSON, and other formats. It includes tools for sensitivity analysis, pattern matching, model testing, and parameter estimation. While primarily a software library, it also contains research examples, paper figure reproduction scripts, and experimental data, making it a hybrid project with a strong application core. The codebase is well-structured with clean abstractions (simulator base class, exporter registry, composable filter predicates) and extensive test coverage, though it carries some technical debt from deprecated modules and external tool dependencies.