Though public opposition to same-sex marriage seems reasonably stable nationally, support in California has grown substantially in the past two decades. Using data from six Field Polls of Californians since 1985, we explore the roots of that growth in individual attitude change and population changes. Cohort replacement can explain half the growth. Although all groups of Californians say that they have become more accepting of homosexual relations since they turned 18, the pattern is strongest for liberals, Democrats, and the less religious. These groups have also become much more supportive of same-sex marriage, while conservatives, Republicans, Protestants, and African-Americans appear at least as opposed today as they were two decades ago.