Consider two situations. First, toxicity increases with dose. Second, in addition, one considers efficacy, and assumes efficacy increases with dose – so except at the extremes, the probability of efficacy without toxicity (success) will increase up to a point at which the toxicity is great enough to cause it to turn down. In the first situation, one typically seeks to identify a dose with a prescribed toxicity rate; in the second situation, one seeks to identify the dose that maximizes the probability of success. These goals can be posed in terms of estimation or dose selection, as a finite sample space of doses is typically required. Three classes of procedures that differ in many fundamental ways are described: up-and-down, adaptive optimal and best intention designs.