Drugs, like other reinforcers, can vary in their relative abilities to support operant responding. Considerable research has been designed to obtain useful measures of a given drug's or dose's "reinforcing efficacy" and to identify the ways in which a variety of behavioral and pharmacological variables impact these measures. Progressive-ratio schedules of drug delivery generate an index of a drug's or dose's reinforcing efficacy (the breaking point) and are being used increasingly as tools in the analysis of drug self-administration. Progressive-ratio schedules of drug delivery have been used to characterize the effects of pretreatment drugs, lesions, drug deprivation, physical dependence, and repeated non-contingent drug exposure on breaking points. Behavioral factors, including food restriction and electric shock, and organismic factors, including gender and strain, have also been investigated using progressive-ratio schedules of drug delivery. To the extent that breaking points provide an index of reinforcing efficacy, these studies demonstrate that a wide range of variables can influence the reinforcing efficacy of self-administered drugs. The objectives of this review are to critique existing research themes, outline potential limitations of progressive-ratio procedures, and to suggest potentially fruitful uses of these procedures in future research.