The authors report on the benefits and costs of California’s new mandatory-sentencing law, which provides for progressively longer sentences with an increasing number of prior convictions for serious felonies. The authors find that the new law, if fully implemented, will decrease serious crime committed by adults by about 28 percent at a cost of an extra $5.5 billion a year. Alternatives that narrow the law’s application result in a lower benefit but an even greater reduction in costs. The authors were also able to devise an alternative that resulted in the same crime-reduction benefit for lower cost. The authors conclude that the state budget cuts required to fund the new law will be so great that it is unlikely to be fully implemented.