  The main result of this paper, called the Avalanche Principle (AP), relates
the expansion of a long product of matrices with the product of expansions of
the individual matrices. This principle was introduced by M. Goldstein and W.
Schlag in the context of ${\rm SL}(2,\mathbb{C})$ matrices. Besides extending
the AP to matrices of arbitrary dimension, possibly non-invertible, the
geometric approach we use here provides a relation between the most expanding
(singular) directions of such a long product of matrices and the corresponding
singular directions of the first and last matrices in the product. The AP along
with other estimates on the action of matrices on Grassmann manifolds will play
a fundamental role in [6] to establish the continuity the Lyapunov exponents
and of the Oseledets decomposition for linear cocycles.
  This is the draft of a chapter in our forthcoming research monograph [6].
