This is the original text of the author's thesis, defended December
18, 2002.
The main result of the thesis is the construction of an equivalence of
category between Rost cycles modules over a perfect field k and an
unstable version of the category of homotopy invariant sheaves with
transfers over k (the latter was introduced by Voevodsky). Meanwhile
the construction of the first arrow of the equivalence has been
published separately in Mathematische Zeitschrift, volume 252, number
2.
An important consequence of this equivalence is a new proof of
Voevodsky's theorem on the homotopy invariance of the cohomology of a
homotopy invariant sheaf with transfers over a perfect field.
In order to show the independance of our proof with that of Voevodsky,
we had to establish separately all the basic facts about sheaves with
transfers. This is done over a regular base of equal
characteristic. Since the defense of the thesis, it was possible to
remove this latter hypothesis. This new work on finite correspondances
over a regular base can be found in a separate preprint K-theory preprint
archive 0765.