Delay discounting represents the extent to which consequences, or outcomes, decrease in effectiveness to control behavior as a function of there being a delay to their occurrence. Higher rates of delay discounting are often operationalized as an index of impulsivity, and as such impulsive discounting may hold considerable potential for understanding fundamental behavioral processes associated with a range of problematic behaviors - including drug use and pathological gambling. This paper first provides a review of several assessment methods used in delay-discounting research with humans. Following, the delay-discounting literature related to drug use and gambling is reviewed. Consistencies across this literature are identified; and future research directions are discussed, which include (a) improving methods of assessment for delay discounting and (b) moving drug-use research progressively to causal interpretations, with high rates of delay discounting either predisposing to drug use or resulting from drug use itself.