During its life cycle a bacterium grows, replicates its DNA and divides into two daughter cells. For the two daughter cells to be viable, each one has to contain one DNA molecule, and thus the cell has to divide in the middle, between the DNA molecules. One mechanism that regulates the central placement of the division site in rod-shaped E. coli is the Min-protein system, which prevents division near the cell ends. A surprising discovery in recent years is that the Min system is an oscillator involving wholesale shifts of proteins from one end of the cell to the other. I will present a model of the Min system, using only known properties of the proteins, which reproduces the formation of a ring of MinE proteins, polar growth of the MinD end caps, dependence of the oscillation period on protein concentrations, and the "zebra stripe" oscillations in filamentous cells. Finally, I will discuss the relation of the model to the recent finding of oscillatory helical structures of the Min proteins.