The development, factor structure, and validity of the Inventory of Drug-Taking Situations (IDTS) is described. This 50-item self-report questionnaire, which is an extension of the Inventory of Drinking Situations (Annis, 1982. Inventory of drinking situations; Annis, Graham & Davis, 1987, Inventory of drinking situations (IDS): User's guide), is designed to assess the situational antecedents to use of a wide range of drugs of abuse. The IDTS consists of 8 subscales that measure a client's substance use in the 8 situations identified in the work of Marlatt (1978, Alcoholism: New directions in behavioral research and treatment): Unpleasant Emotions, Physical Discomfort, Pleasant Emotions, Testing Personal Control, Urges and Temptations to Use, Conflict with Others, Social Pressure to Use, and Pleasant Times with Others. Reliability, factor structure and validity of the IDTS were assessed on 699 clients admitted to the Addiction Research Foundation's treatment facility in Toronto. The IDTS was shown to have reliable subscales. The IDTS total score correlated with self-ratings of the severity of the clients' substance use problem, and with retrospective reports of frequency of use (drugs) and quantity of use (alcohol), years of usage, and severity of dependence. Confirmatory factor analysis was used to test the fit of the data to Marlatt's model of substance use situations, as well as to several alternative models. The goodness-of-fit indicators suggested that the best fit for the data was an 8-factor model corresponding to the 8 subscales based on the Marlatt categories. Evidence was presented that the 8 subscales can be further grouped into 3 second-order factors: (1) negative situations, (2) positive situations, and (3) temptation situations. The negative situation subscales of Unpleasant Emotions, Conflict with Others and Physical Discomfort were found to be correlated with the SCL-90R Depression scale. Interpersonal Sensitivity scale and Somatization scale respectively. The positive social situation subscales of Pleasant Times with Others and Social Pressure to Use were found to be negatively correlated with percentage of time using alone, and positively correlated with pressure from friends and family to use. On the States of Change Readiness and Treatment Eagerness Scales (SOCRATES), clients at the precontemplation stage of change had relatively flat, undifferentiated IDTS profiles (i.e. little scatter of subscale scores), whereas clients in the determination stage had the most differentiation in their IDTS subscale scores. Excellent comparability was found for alcohol clients between the IDS and the IDTS. Evidence was also presented for adequate comparability between the computer and paper and pencil administrative formats of the IDTS.