This chapter introduces the cognitive modeling of human performance in process design and modification as a method to justify the mental load of work design. It covers some human performance modeling (HPM) tools that incorporate cognitive modeling for work system design and modification. The term HPM is used to define systems that simulate human decision making and actions, which are basically synonymous with cognitive simulation and artificial intelligence. Methods for analyzing human performance in the design stage include prototyping, human-in-the-loop (HITL) simulation, and human-out-of-the-loop (HOOTL) simulation. An HITL simulation is a modeling framework that involves human interaction. HITL simulations give researchers and analysts the ability to study the complexities of a system with human involvement from a holistic perspective. Task network modeling is an approach to modeling human performance that organizes task sequences in a manner to represent the human-machine system.