The quality of experimental studies substantially depends on their design, which needs to be specified before data collection. The demand for optimal designs in psychological research and related fields has grown in recent years, not least due to complex and expensive methods such as neuroimaging techniques becoming standard procedures, which has increased the need to control the time and cost involved. Optimal design is also extremely valuable for large-scale assessments and adaptive testing. Beyond that, practically every psychological study may benefit from optimal design, as the reduction of experimental effort often amounts to 20-40%. Still, textbooks on experimental design usually do not cover optimal design, the existing literature on optimal design is often written rather technically and requires profound statistical knowledge, and software packages frequently used in psychology provide few opportunities for generating optimal designs. This volume provides an introduction to optimal design and an overview on new developments and applications of optimal design to several important topics in psychology, including multilevel analysis, event history analysis, functional magnetic resonance imaging (fMRI) studies, and optimal test assembly. All contributions in this compilation have been written with express attention to high readability. They only presuppose statistical knowledge as typically provided by the BSc and MSc study of psychology.