When complex public health and health services interventions are implemented in real-world settings, adaptations should be expected, embraced, and studied rather than suppressed and ignored. A substantial amount of recent research has been conducted on the assessment of some types of adaptations, and interest in guiding adaptations to both interventions and implementation strategies is growing. However, there is still a need to investigate the optimal ways to systematically and pragmatically document, analyze, and iteratively guide adaptations as well as to measure the impact of those adaptations on implementation and effectiveness outcomes. This article reviews key findings from the adaptations assessment literature, frameworks to guide classification of adaptations, and methodologies to study adaptations and their impact. We summarize research from diverse settings and populations from public health and health services research on the use of these methodologies and make recommendations for research, practice research, and practice.