Title: Weighted Clustering Abstract: Clustering is a central unsupervised learning task with a wide variety of applications. However, in spite of its popularity, it lacks a unified theoretical foundation. Recently, there has been work aimed at developing such a theory. We discuss recent advances in clustering theory, with an emphasis on a recent framework for clustering algorithm selection. Faced with a concrete clustering task, a user needs to choose an appropriate clustering algorithm. Clustering algorithms vary widely in their input-output behaviour; consequently, understanding these differences is essential for making this choice. Studying clustering in a weighted setting leads to an elegant classification that highlights fundamental differences between popular clustering techniques. Weighted clustering is a natural generalization of the common clustering framework, where every element is associated with a real-valued weight. We introduce three fundamental categories that describe how an algorithm can respond to weights, and study well-known clustering methods with respect to these categories. Among the algorithms studied, we observe a positive correlation between an algorithm's popularity and its weight-sensitivity. Joint work with Shai Ben-David, Simina Branzei, and David Loker.