Zintl compounds have recently emerged as promising thermoelectric materials due to their rich chemistry and structural complexity. Zintl compounds are defined as valence precise intermetallic phases in which electropositive cations donate electrons to covalently bonded polyanions. In the search for and development of new thermoelectric materials, the vast array of structures and chemical environments found among Zintl phases is an ideal place to begin. This chapter outlines the principles necessary to select, synthesize, and optimize such phases for thermoelectric applications. In the development of high zT thermoelectric materials, there are two key material requirements: low lattice thermal conductivity and chemically tunable electronic properties. The structural complexity characteristic of Zintl compounds leads to glass-like lattice thermal conductivity, making them ideal thermoelectric materials. This complexity also provides numerous opportunities for tuning electronic properties; while substitution of aliovalent elements yields direct control of the electronic carrier concentration, extensive isovalent substitutions can be used to fine-tune thermal and electronic properties by targeted modification of the bonding environment. By exploiting these principles, several excellent Zintl thermoelectric materials have already been developed, highlighting the future potential of these fascinating and complex materials for thermoelectric applications.