Abstract: A back flow preventer valve assembly for protecting fluid supply systems against back flow of contaminating fluids includes a pair of serially connected check valves positioned within respective housings which are connected together and to inlet and outlet fittings by slidable sleeve-type couplings. The couplings are sealed to the fittings and to the valve housings by resilient annular gaskets, preferably O-rings. Tie bolts extend longitudinally from the inlet fitting coupling to the outlet fitting coupling to retain the assembly together. This assembly provides for interchangeability of components, easy removal of the valve units from the line in which they are connected without disconnection of the inlet and outlet fittings, and for easy replacement or substitution of a fitting. Each check valve includes a poppet assembly of advantageous modular construction, with components easily replaceable.