From the glitzy hotel casinos of Las Vegas to the illegal pai gow parlors of New York’s Chinatown, more than 51 million Americans—a full quarter of all those over 21—visited a casino last year. And the number is growing. More states are legalizing gambling, and the casinos are becoming more sophisticated.

Once the casino industry was firmly established in Nevada, it was relatively easy for it to spread across America. But for decades, gambling was illegal almost everywhere else, and it took forty-seven years before a second American state loosened its antigambling laws.

The modern casinos have become incredibly complex, both in terms of the games offered and their security measures. Elaborate surveillance systems offer a high-tech eye-in-the-sky, with cameras watching every table, window and doorway. The cameras can be adjusted to focus on suspicious patrons by personnel in a separate room filled with banks of security monitors. Casinos have also developed more specialized surveillance techniques, with casino employees trained to spot subtle cues that might indicate cheating.

Of course, the casinos’ ultimate goal is to make money. That’s why they have a built-in advantage on each game, usually no more than two percent. Over time that edge rakes in enough profits to pay for the hotels, fountains, pyramids and towers they build to attract players. And it makes it possible for them to justify charging a “vigorish” or a “rake,” as the industry calls it, on each bet.