This chapter is about how metazoan animals swim in the diversity of aquatic environments—the oceans, brackish waters, and fresh waters. The wide variety of animals discussed spend at least significant parts of their respective life histories moving through waters that can vary widely in temperature, solute content, and hydrostatic pressure and therefore in density, viscosity, buoyancy, and surface tension. They also vary widely in flow characteristics such as velocities of bulk flows and levels and scales of vorticity (turbulence) (Denny 1988, 1993; Kaiser et al. 2011; additional references in Chapter 1).