  This work consists two parts. In the first part, we completely study the heat
equation method of Menikoff-Sjostrand and apply it to the Kohn Laplacian
defined on a compact orientable connected CR manifold. We then get the full
asymptotic expansion of the Szego projection for (0,q) forms when the Levi
formis nondegenerate. This generalizes a result of Boutet de Monvel and
Sjostrand for (0,0) forms. Our main tool is Fourier integral operators with
complex valued phase functions of Melin and Sjostrand. In the second part, we
obtain the full asymptotic expansion of the Bergman projection for (0,q) forms
when the Levi form is non-degenerate. This also generalizes a result of Boutet
de Monvel and Sjostrand for (0,0) forms. We introduce a new operator analogous
to the Kohn Laplacian defined on the boundary of a domain and we apply the heat
equation method of Menikoff and Sjostrand to this operator. We obtain a
description of a new Szego projection up to smoothing operators. Finally, by
using the Poisson operator, we get our main result.
