Single-case research designs are often applied within school psychology. This article provides a critical review of the scientific merit of both concurrent and nonconcurrent multiple baseline (MB) designs, relative to their capacity to assess threats of internal validity and establish experimental control. Distinctions are established between AB replications and nonconcurrent multiple baseline designed studies using the initial conception proposed by P.J. Watson and E.A. Workman (1981). Despite some previously pessimistic evaluations of nonconcurrent multiple baseline designs, the findings of this review suggest that various threats of internal validity can be assessed and ruled out using either concurrent or nonconcurrent MB designs. It seems that nonconcurrent designs can be used to assess the intervening effects of history, but might be more prone to threats of mortality. These and other threats of internal validity are reviewed and recommendations are provided. (Contains 2 tables and 1 figure.)