Chemical kinds are generally treated as having timelessly fixed identities. Biological kinds are generally treated as evolved and/or evolving entities. So what kind of kind is a biochemical kind? This article defends the thesis that biochemical molecules are clustered chemical kinds, some of which—namely, evolutionarily conserved units—are also biological kinds. On this thesis, a number of difficulties that have recently occupied philosophers concerned with proteins and kinds are shown to be either resolved or dissolved. 1 Introduction2 Conflicting Intuitions about Kinds (...) of Proteins3 Against Permissive Pluralism4 Against Nominalism, Toward a Duality of Kinds5 Biological Kinds, Chemical Kinds, and Their Relations6 Implications for Biological Individuals7 Implications for Monism and Scientific Practice. (shrink)