  We present a general conceptual framework for self-organized criticality
(SOC), based on the recognition that it is nothing but the expression,
''unfolded'' in a suitable parameter space, of an underlying {\em unstable}
dynamical critical point. More precisely, SOC is shown to result from the
tuning of the {\em order parameter} to a vanishingly small, but {\em positive}
value, thus ensuring that the corresponding control parameter lies exactly at
its critical value for the underlying transition. This clarifies the role and
nature of the {\em very slow driving rate} common to all systems exhibiting
SOC. This mechanism is shown to apply to models of sandpiles, earthquakes,
depinning, fractal growth and forest-fires, which have been proposed as
examples of SOC.
