This chapter provides a review of non-take-up figures from a number of western countries. It presents theoretical models for understanding the underlying causes of non-take-up, and explores some results of an empirical application of the models. Most British research on non-take-up has been directed mainly to means-tested benefits. In the Netherlands the non-take-up of housing benefit has attracted attention over the longest period of time, mostly as part of broader research on housing needs and problems. In the USA a range of means-tested federal, state and local social welfare programs exists, in which non-take-up, or nonparticipation as it is usually called in American literature. Reviews of non-take-up research show that traditionally the causes of non-take- up of social security benefits are studied at the client-level only. The chapter explains new approaches to the causes of nontake-up, tackling both the confinement to the client level, as well as the need for a more consistent and valid client-level model.