The fundamental aspects of isotropic turbulence are reviewed in order to gain a better insight into the physical processes of turbulence. After first reviewing the Kolmogorov energy spectrum and the energy cascade, the Kolmogorov hypothesis of local isotropy is discussed in depth. Then, the detailed physical processes involving energy transfer and interacting scales in isotropic turbulence, including triad interactions, are reviewed. The inertial range and self-similarity are also discussed along with the response of the small scales to large-scale anisotropy and the final stages of the decay process. Results from direct and large-eddy simulations of isotropic turbulence—including a discussion of subgrid scale modeling—are then discussed in detail to illustrate these points. The article closes with a review of self-preservation in isotropic turbulence and a discussion of the prospects for future research. It contains 155 references.