This chapter will show why thinking of psychiatric nosology on the model of biological classification has the potential to be highly misleading. Psychiatric classification is importantly different from, and in many respects far more problematic than, biological classification. This chapter provides an overview of just some of the issues involved. It is divided into three, somewhat interconnected, sections: puzzles in the logic of classification, puzzles in the metaphysics of classification, and puzzles in the ethics and pragmatics of classification.