Long-distance, high-capacity transmission using a light beam in a beam waveguide is an attractive project for optical communications. However, the transmission of energy in the form of a beam gives rise to a new problem, that is, the stability of a sharp beam. In this paper, the stability problems of a Gaussian beam in a beam waveguide are presented. The deflection and the spot-size variation of a beam in the waveguide with known imperfections, such as transverse displacements of lenses, variation in focal length, and so on, are studied. If the transverse displacements of the lenses are periodic in space, instability of the beam path may occur. The unstable condition is derived. The variation of the spot size at the lenses is obtained in terms of small perturbations of the ideal waveguide. If the incident beam from a laser is not matched to the mode of the waveguide, the spot size at the lenses varies periodically. Statistical results for the models of a waveguide with random lenses, having random transverse position, random spacing, and random focal length, are also given.