The authors thank Philip Armitage, Mitchell Begelman, Andy Fabian, David Garofalo, Kazushi Iwasawa, Julia Lee, Herman Marshall, Cole Miller, Jon Miller, Norbert Schulz, Joern Wilms and Andrew Young for their careful reading, and insightful comments, of various drafts of this article. We are particularly grateful to the referee, Eric Agol, whose report led to a substantial improvement of the manuscript. In addition, we also express our appreciation to Tomaso Belloni, Omer Blaes, Jim Chiang, Paolo Coppi, Stephane Corbel, James Dove, Rob Fender, Julian Krolik, Tom Maccarone, Phil Maloney, and Katja Pottschmidt for stimulating discussions and help in preparing some of the figures for this article. We acknowledge and thank NASA for financial support under grants NAG5-10083 (CSR) and SV-61010 (MAN).