  We study crossed products of arbitrary operator algebras by locally compact
groups of completely isometric automorphisms. We develop an abstract theory
that allows for generalizations of many of the fundamental results from the
selfadjoint theory to our context. We complement our generic results with the
detailed study of many important special cases. In particular we study crossed
products of tensor algebras, triangular AF algebras and various associated
C*-algebras. We make contributions to the study of C*-envelopes,
semisimplicity, the semi-Dirichlet property, Takai duality and the Hao-Ng
isomorphism problem. We also answer questions from the pertinent literature.
