The following treatise presents the scientific data base used by Washington Department of Fisheries (WDF) fish biologists and fish passage engineers to establish maximum approach velocity, mesh size, and screen orientation criteria for fish screen facilities that must protect chinook and coho salmon fry. Before proceeding with a discussion of the validity of the design criteria, however, it is important that the goals and objectives of the WDF juvenile fish protection program be explicitly stated. Obviously, fish screening facilities are constructed to protect juvenile fish from mortality associated with water diversions. This basic goal can be broken into two primary objectives. First, a fish screen facility must physically exclude the critical species and life stage from the water diversion. Secondly, the facility must safely and rapidly return the fish to the parent water body. Approach velocity, screen orientation and bypass criteria address this objective. Any screen facility that fails to adequately perform both functions is not worth constructing.