We suggest that the concept of self-organized criticality (SOC) is relevant for understanding the processes underlying earthquakes. Earthquakes are an important part of the relaxation mechanism of the crust which is submitted to inhomogeneous increasing stresses accumulating at continental-plate borders. The SOC concept then implies that earthquakes in turn organize the crust both at the spatial and temporal levels. This idea allows to rationalize observations on occurrences and magnitudes of earthquakes. Variants of SOC as well as a novel type of dynamics based on "waiting times" are discussed within a mean-field-like approach which shows the existence of 1/f noise in the time gap between large earthquakes. The corresponding long-time correlations have important implications for the statistical long-time forecasting of earthquakes.