Clinical research methods have two objectives: (1) to build a scientific foundation for understanding normal and abnormal human behavior and (2) to determine which interventions are effective in changing problematic behavior and maladaptive emotional states and the conditions in which these interventions are effective. Key quantitative and qualitative research designs and techniques used to accomplish these objectives are described in this entry. These include experimental and quasi-experimental designs, observational designs, qualitative designs such as interviews and qualitative analysis of open-ended responses or databases, single-subject designs, and case studies. Case studies are often not considered empirical but have their place in clinical research. Therefore, their role is described in the context of clinical research methods.