A single-subject design often used to compare the effectiveness of two or more independent variables (like treatment programs) is the multielement (alternating treatments or simultaneous treatments) design. Variants of this design approximate the concurrent comparison of the effects of two or more variables (or levels of variables) by programming the variables (or levels) in rapid alternation, typically across or within daily sessions. Properly combined with conventional reversal designs, these designs can also display a variety of interaction effects, some of them worrisome, others highly desirable for the future development of the field. A worrisome model is the possibility that when Treatment B alternates rapidly with Treatment C, the effects of each will not be the same as when each is the only treatment used. A desirable model is the use of the multielement design as a fast-paced component of an otherwise conventional reversal design examining contextual control of some relationship: the possibility that some behavior responds differently to Controlling Variables A and B in Context X than in Context Y. This second possibility opens single-subject designs to the more efficient examination of all interactive effects and is highly desirable, considering the prevalence and importance of interactions in determining the limits and the generality of currently understood behavioral phenomena.