[The original version, submitted Feb 3, 2005, has been slightly
revised Oct 14, 2005.]
In this paper we study the "slice filtration" defined by effectivity
conditions on Voevodsky's triangulated motives, and apply it to obtain
spectral sequences converging to their motivic cohomology. These
spectral sequences are particularly interesting in the case of mixed
Tate motives as their E_2-terms then have a simple description. We
apply this in particular to get spectral sequences converging to the
motivic cohomology of a split connected reductive group.