Muscle phenotype is the result of interactions between an animal’s genotype and the environment. Asingle genotype can result in multiple phenotypes depending on environmental in«uences in what is known as phenotypic plasticity (Rezende et al., 2005) (Figure 1.1). In adult Œshes, phenotypic plasticity occurs seasonally or with movement across environmental clines, and these changes are thought to be reversible. In contrast, environmental in«uences during ontogeny can lead to distinct and irreversible muscle phenotypes as an expression of different developmental trajectories (Spicer and Burggren, 2003) (Figure 1.1, Section 1.4). The importance of early life experience on the muscle phenotypes expressed later in life has long been recognized, but only recently has the in«uence of developmental exposures on muscle plasticity and swimming performance in adult Œsh been explicitly demonstrated (Scott and Johnston, 2012) (Figure 1.6). Skeletal muscles play numerous important roles in Œshes, from powering locomotion to regulating whole-body metabolic homeostasis (Craig and Moon, 2011). Since muscles constitute asigniŒcant proportion of aŒsh’s body mass, the ability to modulate the mechanical and metabolic properties of this tissue, in-line with environmental optima, is an important feature allowing species to adjust to changing environmentalconditions. Phenotypic plasticity of the muscle is thus an important mechanism assuring that ecologically relevant tasks can still be performed in the face of environmental, mechanical, and/or energetic stress (Figure 1.1).