Clustering techniques are used to arrange genes in some natural way, that is, to organize genes into groups or clusters with similar behavior across relevant tissue samples (or cell lines). These techniques can also be applied to tissues rather than genes. Methods such as hierarchical agglomerative clustering, k -means clustering, the self-organizing map, and model-based methods have been used. This chapter focuses on mixtures of normals to provide a model-based clustering of tissue samples (gene signatures) and gene profiles.