%0 Journal Article %T Abstract decomposition theorem and applications %A Rami Grossberg %A Olivier Lessmann %J Mathematics %D 2005 %I arXiv %X Let K be an Abstract Elementary Class. Under the asusmptions that K has a nicely behaved forking-like notion, regular types and existence of some prime models we establish a decomposition theorem for such classes. The decomposition implies a main gap result for the class K. The setting is general enough to cover \aleph_0-stable first-order theories (proved by Shelah in 1982), Excellent Classes of atomic models of a first order tehory (proved Grossberg and Hart 1987) and the class of submodels of a large sequentially homogenuus \aleph_0-stable model (which is new). %U http://arxiv.org/abs/math/0509707v1