The evolution of altruism is often referred to as the central paradox of evolutionary biology. My colleagues and I have been studying microbial altruism in E. coli. Altruism in this system involves a cell secreting a substance called beta-lactamase, which breaks down antibiotics, and protects not just the cell secreting this substance, but all cells in the general vicinity. We've done experiments that show that producing beta-lactamase is expensive and cells that don't pay these costs-- cheaters cells --grow more quickly than cells that do (when no antibiotics are around). And yet, these secreting altruists coexist side by side with their cheating fellow cells. I'll talk about experiments that help us understand why, briefly touch on some computer simulations that model microbial altruism, and then discuss work we have done that extends microbial altruism and cheating to interspecific interactions between E. coli and Salmonella cells.