Measures of the eddy scale sizes in turbulent flow. The separation between the largest and smallest sizes is determined by the Reynolds number. The largest length scales are usually imposed by the flow geometry, for example, the boundary layer depth. Because turbulence kinetic energy is extracted from the mean flow at the largest scales, they are often referred to as the “energy-containing” range. The smallest scales are set by the viscosity and the rate at which energy is supplied by the largest-scale eddies. Intermediate between these scales are the inertial subrange scales for which turbulence kinetic energy is neither generated nor destroyed but is transferred from larger to smaller scales. Smaller-scale eddies are generated from the larger eddies through the nonlinear process of vortex stretching. Typically, energy is transferred from the largest eddies to the smallest ones on a timescale of about one large- eddy turnover. There are standard turbulence length scales for each of the eddy scale sizes; integral length scales for the energy-containing eddies, Taylor microscale for the inertial subrange eddies, and Kolmogorov microscale for the dissipation range eddies.