Contrary to what traditional asset pricing would imply, a strategy that bets against beta, by going long in low beta stocks and short in high beta stocks, tends to outperform the market. We consider a market in which diversity is maintained, i.e. no single stock can dominate the entire market, and we show that beta-arbitrage strategies mechanically out-perform the market portfolio. We provide empirical support to our explanation on equity country indices, equity sectors, individual stocks, and stock portfolios. Finally, we show how to construct optimal beta- arbitrage strategies that maximize the expected return relative to a given benchmark.