Mathematical models help scientists, developers, and engineers understand processes, phenomena, and designs of reacting systems. The Chemical Reaction Engineering Module, an add-on to the COMSOL Multiphysics® software platform, provides user interfaces for creating, inspecting, and editing chemical equations, kinetic expressions, thermodynamic functions, and transport equations. After developing a validated model, it can be used for studying different operating conditions and designs of reacting systems and transport phenomena. Solving the model equations over and over for different inputs leads to a true understanding of the studied system. Additionally, the Chemical Reaction Engineering Module, along with other tools in COMSOL Multiphysics®, provides state-of-the-art mathematical and numerical methods adapted for optimization and parameter estimation of chemical systems.