Blackjack, or 21, is among the most popular casino table games. Since unlike most other games of chance, successive hands of blackjack are not independent, the mathematics behind blackjack is at once more complicated and more interesting than for games like craps or roulette, and there can be times during play when the gambler has an edge over the casino. This talk will briefly review the rules of the game and then describe some of the calculations--both theoretical and experimental--that led to blackjack basic strategy and the advantages derived from card counting.