Reusability is widely believed to be a key to improving software development productivity. However, in practice, effective reuse is more an achievement of good development environments than a strategy for software development. It seems natural that a reduction of the effort needed for reuse should have a positive impact on reuse. Among the various lines of attack that may lead to a reduction of the reuse effort, we address the problem of finding and understanding reusable functionality in an object-oriented software environment. This paper presents the Affinity Browser, a selection and exploration tool based on the notion or affinity between objects. The user is presented with a two dimensional display where the objects are displayed in such a way that their relative positions convey their affinity i.e., objects lying closer together are more strongly related than objects lying farther apart. The browser provides for different views of the relationships among objects. Each view is based on a different measure of affinity and thus portrays a different relationship. We discuss the rationale behind the Affinity Browser tool and discuss the way users can take advantage of it to understand the functionality provided by a set of classes. The Affinity Browser's abilily to help understanding relationships between objects will be illustrated by two examples of view construction.