Positioning of the midcell division plane within the bacterium E. coli is controlled by the min system of proteins: MinC, MinD, and MinE. These proteins coherently oscillate from end to end of the bacterium. We present a reaction-diffusion model describing the diffusion of min proteins along the bacterium and their transfer between the cytoplasmic membrane and cytoplasm. Our model spontaneously generates protein oscillations in good agreement with experiments. We explore the oscillation stability, frequency, and wavelength as a function of protein concentration and bacterial length.