Book excerpt: Human-in-the-Loop Simulations is a compilation of articles from experts in the design, development, and use of human-in-the-loop simulations. The first section of the handbook consists of papers on fundamental concepts in human-in-the-loop simulations, such as object-oriented simulation development, interface design and development, and performance measurement. The second section includes papers from researchers who utilized HITL simulations to inform models of cognitive processes to include decision making and metacognition. The last section describes human-in-the-loop processes for complex simulation models in trade space exploration and epidemiological analyses. Human-in-the-Loop Simulations is a useful tool for multiple audiences, including graduate students and researchers in engineering and computer science.