Single-case research designs have emerged as a major methodological strategy in some areas of social science research. Yet, single-subject research is plagued by a number of methodological, conceptual, and philosophical problems that have a bearing on the efficacy of these strategies in applied settings. In this article we review briefly some methodological issues in single-case design, including factors such as baseline trends, variability in the data, and duration of phases. In addition, conceptual issues include the range of outcome questions that single-case designs are designed to address as well as establishing the generalizability of findings. Beyond these issues, a number of pitfalls and hassles are likely to emerge, including philosophical objections to single-case research design, replication problems, measurement paradigms that seem incompatible with design assumptions, and the integration of single-case designs into practice.