Although there is consensus that a substantial number of individuals are affected by the addiction-type problems of family members, there has not yet been a mutually agreed prevalence estimate of the dimension of the problem. It can be assumed that the number of addiction-affected family members (AFMs) outweighs the number of individuals with addiction-type problems, and that addiction in the family is a substantial contributor to the global burden of adult ill-health. However, given that most studies on adult AFMs have been conducted in small non-representative samples, it is unclear how generalizable findings from such small-scale studies are, with regard to the population and their public health impact. This chapter outlines different approaches to quantify the prevalence of AFMs and associated methodological problems. Prevalence estimates vary considerably according to the definition of relationship status and problem severity. Most of these estimates are restricted to problems stemming from one substance (and, to a lesser extent, single behavioural addiction-type problems). The public health relevance of addiction-type problems in the family and implications for services aiming to reduce the burden of AFMs are discussed.