Divorce, nonmarital childbearing, and cohabitation are reshaping family experience in the U.S. Because of these changes, our traditional definitions of "families" decreasingly capture the social units of interest. We have noted how a significant proportion of officially defined single-parent families are, in actuality, two-parent though unmarried families. We make a similar argument in the present paper that the definition of stepfamilies must be expanded to include cohabitations which involve a child of only one partner, and explicitly recognize that stepfamilies include those formed after nonmarital childbearing as well as after separation or divorce. We find that cohabitation and nonmarital childbearing have been important aspects of stepfamily experience for at least two decades, and that this is increasingly so. To define stepfamilies only in terms of marriage clearly underestimates both the levels and the trend in stepfamily experience: when cohabitation is taken into account, about two-fifths of all women and 30 percent of all children are likely to spend some time in a stepfamily.