The classical two-arm randomized clinical trial (RCT) is designed to test the efficacy or effectiveness of an intervention, which may consist of one or more components. However, this approach does not enable the investigator to obtain information that is important in intervention development, such as which individual components of the intervention are efficacious, which are not and possibly should be removed, and whether any components interact. The Multiphase Optimization Strategy (MOST) is a new framework for development, optimization, and evaluation of interventions. MOST includes the RCT for purposes of evaluation, but inserts a phase of research before the RCT aimed at intervention optimization. The optimization phase requires one or more separate trials similar in scope to an RCT, but employing a different experimental design. The design of the optimization trial is selected strategically so as to maximize the amount of scientific information gained using the available resources. One consideration in selecting this experimental design is the type of intervention to be optimized. If a fixed intervention, i.e. one in which the same intervention content and intensity is provided to all participants, is to be optimized, a factorial experiment is often appropriate. If an adaptive intervention, i.e. one in which intervention content or intensity is varied in a principled manner, is to be optimized, a sequential multiple-assignment randomized trial (SMART) is often a good choice. The objective of this article is to describe MOST and the scientific rationale for its use; describe two current applications of MOST in emergency medicine research, one using a factorial experiment and the other using a SMART; and discuss funding strategies and potential future applications in studying the care of individuals with acute illness, injury, or behavioral disorders.