A theoretical study is made of attenuation of ultrasound in cylindrical and rectangular waveguides. The systems treated consist of a piston source radiating into a bounded, inviscid fluid and a receiver of the same size as the source. Expressions for receiver output are derived, and acoustic loss as a function of path length is computed for various source and waveguide configurations. Boundary conditions corresponding to both perfectly rigid and perfectly free walls are considered. The waveguide approach is extended to the limiting case of radiation into an unbounded medium, and results are given for both square and circular systems. In the latter case the curve of loss versus path length agrees well with that previously obtained using more conventional diffraction theory.