Motivated by Khovanov's categorification of the Jones polynomial, we study functors F from thin posets P to abelian categories A. Such functors F produce cohomology theories H * (P, A, F ). We find that CW posets satisfy conditions making them particularly suitable for the construction of such cohomology theories. We consider a category of tuples (P, A, F, c), where c is a certain {1, −1}-coloring of the cover relations in P , and show the cohomology arising from a tuple (P, A, F, c) is functorial, and independent of the coloring c up to natural isomorphism. Such a construction provides a framework for the categorification of a variety of familiar topological/combinatorial invariants: anything expressible as a rank-alternating sum over a thin poset.