Publisher Summary One particular subset of heterocyclic natural products is one that contains both nitrogen and sulfur atoms, and over the years, a number of such N,S-containing compounds have been discovered. These natural products include 1,4-thiazines and their di-, tetrahydro, and benzo derivatives. This chapter outlines the wide range of 1,4-thiazines that occur in nature. The 1,4-thiazine containing alkaloids are relatively rare in nature, but they range in structural complexity from simple monocyclic derivatives, such as chondrine, and benzothiazines, such as the aplidinones, to more complex tri- and polycyclic compounds exemplified by ansathiazin and the shermilamines. Thiazole rings also occur in important biologically active natural products such as the epothilones. Benzo-1,4-thiazines also derive from cysteine, often by addition to benzoquinones. The chapter discusses the chemistry of di- and tetrahydro-1,4-thiazines and their s-oxides, benzo-1,4-thiazines, and tricyclic thiazines. It also discusses structurally complex thiazines, where the heterocycle forms a part of a polycyclic system. Many of these natural products also contain quinone chromophores, and in many cases, the corresponding compound lacking the thiazine ring also occurs in nature.