This collection of articles elaborates on various aspects of outcome studies. Outcome studies feed and nurture the field of social work and other human services professions with evidence on what works. Outcome studies are designed to measure and understand the effectiveness of social, behavioral, and educational interventions. The ultimate purpose of social work action is change; that is, intended betterment of living conditions of a client, a family, or a community. Intended outcomes are often carefully considered and targeted when designing an intervention to cause betterment. However, social work interventions take place in real-life situations in which many other factors than the intervention itself are present and interfere to different degrees under different circumstances. In social work and other human services, unintended outcomes are very common and frequently complicate the processes of designing outcome studies and measuring outcomes. Thus, the main challenge of any outcome study is to determine what would have happened in the absence of a professional intervention in order to estimate the outcomes of the intervention program. As noted by Fraser and colleagues in this collection, intervention studies and outcome studies are akin but still different. Outcome studies belong to the general sphere of program evaluation. Intervention research includes phases of designing and developing an intervention program to be explicitly tested within the framework of the intervention research. Conceptualization of data-driven program theory, efficacy testing under strictly controlled conditions, and effectiveness testing in real-life situations, and on a larger scale compose the main steps of intervention research. Outcome studies and intervention research form the backbone of evidence-based practices in social, behavioral, and educational professions. In other words, outcome studies are designed to substantially contribute to the evidence base of social work and other human services. The collection of the articles offered in this special issue is the proceedings of the Stockholm Conference on Outcome Studies of Social, Behavioral, and Educational Interventions sponsored by the National Board of Health and Welfare, the Swedish Council for Working Life and Social Research, the Swedish National Board for Institutional Care, and the Swedish National Institute for Public Health in collaboration with the Hamovitch Center for Science in the Human Services at the School of Social Work, University of Southern California, held on February 7 to 8, 2011. The collection of articles begins with an introductory presentation by Mark W. Fraser and his colleagues and can be read parallel to a handy book, Intervention Research: Developing Social Programs (Fraser, Richman, Galinsky, & Day, 2009). This article describes the core features of outcome research and discusses challenges to designing and conducting robust and compelling outcome studies. As much as outcome studies are essential to evidence-based social work practice, it is equally difficult to conduct successful outcome studies—a problem often revealed during study quality appraisal processes performed by systematic review teams with the Campbell and Cochrane collaborations. Some of the issues raised by Fraser and colleagues are also addressed by Kelly and Moore, who examine the process of building an evidence base for better and more effective public health and mental health services from a British perspective. Kelly, the director of the Centre for Public Health Excellence, located in London, England, offers a special insight into national perspectives on policies of making public health interventions effective and cost-effective. Preoccupation with scientific and technical aspects of outcomes studies may at times blind researchers, decision makers, and professionals when assessing the meaning and ethics of outcome studies. In her contribution, Eileen Gambrill critically assesses ethical aspects of outcome studies, arguing that no research area is more political than the evaluation of outcome studies, and fundamentally challenges outcome researchers to serve social work clients on sound ethical grounds.