The main problem, deforming a subalgebra of a Lie algebra, is treated algebraically, requiring an extensive " development of methods of defining multiplications on Lie algebra cohomolo,gy cochains. Some applications to differential geometry are also presented.To be submitted to Communications in Math. Physics. In this paper, we shall develop the full algebraic formalism necessary to discuss this deformation problem. As can be seen from Ref. 4, this necessitates studying the "multiplicative" structure on the cochains associated with Lie algebra c ohomology . We have delayed presenting this theory because of its complexity, but in this paper we can present a relatively simple independent exposition, and show how it is applied to the interesting deformation problems in a straightforward way. There is considerable overlap in results with work done by A. Nijenhuis and R. Richardson [ 5,6,9,10] . However, the methods presented here are perhaps better adapted to the explicit calculations that are necessary to apply the theory to interesting problems of group representations and differential geometry. Our aim is to show that o! induces a bilinear map, which we also denote by Q, of Cr( 4,) X Cs( 9,) --+Crfs( $3), for each pair (r, s) of non-negative integers. Now, for I' = s = 0, Cr($,) = Vl, C'($I,) = V2, Cr+S(~3) = V3 . We require in this case that a! be the same as the map we are given. We will now proceed by induction on (1: + s), assuming that a! is defined on Cr' X C", for r'+s'