The enormous diversity of form of marine invertebrate larvae has been long recognized (see Young et al., 2002, for a compilation). As larvae represent a transient stage in a species’ life history, their performance in situ has effects on survivorship in the near term (larval life) and carryover effects that are realized in juveniles (e.g., Marshall et al., 2003; Pechenik, this volume); recent work also indicates that such effects may exist across generations (Dupont et al., 2008; Ross et al., 2016). Irrespective of how larval performance is measured (development rate, metabolic rate, growth rate, etc.), it represents the summation of the physiological activities of the individual, the interdependence among cells that make up the whole, and the correct positioning of these cells within the larval body. Thus, the physiological properties of larvae are constrained, in part, by their form, and the phrase, “physiology is the handmaid of anatomy,” although typically applied in other contexts (e.g., human anatomy and physiology; Robinson, 1847), aptly applies to marine invertebrate larvae. The focus of this chapter is to examine the physiology of marine invertebrate larvae from an organismic perspective. The functional attributes of larvae are, in part, a result of the integration of function(s) of their different parts (cells, tissues, and organs). Highlighting the importance of understanding the relationship between structure and function in the context of larval physiology is an intentional focus of this document.