The concept of similarity is central to perception. It underlies processes such as object recognition, comparison, memory retrieval, classification, problem solving, and inductive reasoning, which are crucial to much of human cognitive processing. In the domain of music, similarity plays a vital role in perceptual and cognitive processes such as grouping, segmentation, musical variation, genre recognition, and expectancy. Furthermore, memory and schematization processes in real-time listening rely on the recognition of small figures, or cues, which again is based on the notion of similarity (Deliège, 2001). Therefore, it is evident that understanding of the basic processes underlying perception of musical similarity is necessary for acquiring a deeper comprehension of music perception in general. The articles published in this issue, although dealing with the common topic of similarity, comprise a diversity of different approaches and methodologies, and to provide a synopsis of them is an arduous task. We asked Geraint Wiggins to take on this job, and he was brave enough to agree. In his article, he compares the different approaches presented in this issue, and provides a summary of them in particular in light of crucial issues of modeling such as knowledge and data representation as well as the distinction between explanatory and descriptive models. ...