  The categories of almost modules and almost algebras are introduced as a
convenient setting for the development of Faltings' method of almost etale
extensions. After some preliminaries of general "almost homological algebra" we
construct the almost version of the cotangent complex and we use it to
generalise some results of Faltings on the lifting of almost etale morphisms
and almost etale algebras over nilpotent extensions. We also study the "almost
trace" of an almost flat and almost finitely presented morphism, in particular
we show that the almost trace is (almost) perfect if and only if the morphism
is almost etale. Finally we study some cases of non-flat descent for almost
rings, and establish the invariance of almost etale morphisms under Frobenius.
