The interest group concept is defined in many different ways in the existing literature and a range of different classification schemes are employed. This complicates comparisons between different studies and their findings. One of the important tasks faced by interest group scholars engaged in large-N studies is therefore to define the concept of an interest group and to determine which classification scheme to use for different group types. After reviewing the existing literature, this article sets out to compare different approaches to defining and classifying interest groups with a sample of lobbying actors coded according to different coding schemes. We systematically assess the performance of different schemes by comparing how actor types in the different schemes differ with respect to a number of background characteristics. This is done in a two-stage approach where we first cluster actors according to a number of key background characteristics and second assess how the categories of the different interest group typologies relate to these clusters. We demonstrate that background characteristics do align to a certain extent with certain interest group types but also find important differences in the organizational attributes of specific interest group types. As expected, our comparison of coding schemes reveals a closer link between group attributes and group type in narrower classification schemes based on group organizational characteristics than those based on a behavioral definition of lobbying.