There are many different methods for finding groups in data (cluster analysis), and on many datasets they will deliver different results. How good a clustering is for given data depends on the aim of clustering and on the user's concept of what makes objects "belong together". I will present some approaches to assess the quality of a clustering and to compare different clusterings. Particularly, I will present some indexes that measure various desirable aspects of a clustering such as stability, separateness of clusters etc. Different aims of clustering can be taken into account by specifying which aspects are particularly relevant in the situation at hand.