In this chapter, we discuss the methodology beyond the mathematical modeling of solar water-splitting cells. In particular, we focus on the governing mathematical equations and relationships at the continuum level for mass, energy, light, and species transport and interactions as well as their implementation in a multiscale, multiphysics model. The chapter also discusses the rationale and objectives of continuum modeling including relevant perspective and cell-design case studies that encompass limiting cases. Throughout, possible issues with modeling and their mitigation are introduced so as to help the reader understand the pitfalls and power of modeling.