  Drag reduction by microbubbles is a promising engineering method for
improving ship performance. A fundamental theory of the phenomenon is lacking
however, making actual design quite uphazard. We offer here a theory of drag
reduction by microbubbles in the limit of very small bubbles, when the effect
of the bubbles is mainly to normalize the density and the viscosity of the
carrier fluid. The theory culminates with a prediction of the degree of drag
reduction given the concentration profile of the bubbles. Comparisons with
experiments are discussed and the road ahead is sketched.
