Several surveys of the use of alcohol and illegal drugs in Iceland have been conducted during the past 15 years. Studies of the adult population have been primarily concerned with the use of alcohol, and those conducted among adolescents have also included questions on the use of illegal drugs. Regular surveys on drinking patterns among the adult population have focused on intrasocietal comparability. Some methodological problems associated with longitudinal studies are briefly discussed in this chapter, and the experience derived from the Scandinavian Drinking Survey is considered. Surveys on adolescent drug use have usually been conducted as single projects, and their limited comparability is discussed. Steps are proposed to increase the comparability of future surveys. A core of joint questions is needed across surveys, but each survey should permit the adaptation, deletion, and addition of questions at different times and in different settings. A common stock of questions, including items on frequency and quantity, that can be transformed into units of absolute alcohol in centiliters will make comparison easier. In this connection, the different coverage rates among nations should be taken into account. The frequency of intoxication is also an important factor that is desirable to standardize, but involves further complications. 23 references