Self-organized criticality (SOC) is a theoretical concept that describes the statistics of nonlinear processes. It is a fundamental principle common to many nonlinear dissipative systems in the universe. Due to its ubiquity, SOC is a law of nature, for which we derive a theoretical framework and specific macroscopic physical models. Introduced by Bak, Tang, and Wiesenfeld in 1987, the SOC concept has been applied to laboratory experiments of sandpiles, to human activities such as population growth, language, economy, traffic jams, or wars, to biophysics, geophysics, magnetospheric physics, solar physics, stellar physics, and to galactic physics and cosmology. From an observational point of view, the hallmark of SOC behavior is the power law shape of occurrence frequency distributions of spatial, temporal, and energy scales, implying scale-free nonlinear processes. Power laws are neither a necessary nor a sufficient condition for SOC behavior, because intermittent turbulence produces power law-like size distributions also. A novel trend that is ongoing in current SOC research is a paradigm shift from “microscopic” scales toward “macroscopic” modeling based on physical scaling laws.