Almost all our knowledge about cohabitation in the United States rests on analysis of nationally representative, large-scale surveys. We move beyond this work by drawing on 115 in-depth interviews with a sample of young men and women with recent cohabitation experience. These data allow us to address two issues of central interest to family studies. First, we use our qualitative data to assess the measurement of cohabitation in surveys and the census. We find that current measurement strategies are probably underestimating cohabitation, and we may need to find new ways to measure cohabitation. Second, we employ qualitative findings to address issues relating to how we empirically model union formation. We find that the movement into cohabitation is not akin to marriage. It is often not a deliberate decision. Couples do not appear to be deciding between cohabitation and marriage; rather, their decision seems to center around whether to remain single or cohabit. These results have important implications for our analysis and understanding of cohabitation.