In thinking about the results reported here from the 2000/01 NJPS, it is critical to have a clear-eyed view of how the social landscape in which American Jews find themselves has changed in the post-World War II period. Some of the papers (Phillips and Fishman; Phillips and Kelner) argue that a soft version of rational choice can help to shed light on such key indicators of the contemporary Jewish situation as intermarriage, and I agree that it can. But a choice-based framework is useful only insofar as one has a firm conceptual grip on the range of choices faced by minority individuals, along with their perceived benefits, risks, and costs. If that configuration has changed substantially over time, past research may not give useful guidance, indeed may be fundamentally incommensurate with the findings from new data. The applicability of past models is, quite appropriately, called into question by some of the papers, most notably, Phillips and Kelner. The concept of a social boundary provides a useful way of specifying some of the major changes that have taken place in the last six decades or so, as well as of identifying the value of the American Jewish case for comparative studies. By a boundary, I mean a social distinction that individuals make in their everyday lives and that shapes their actions and mental orientations towards others; it is typically embedded in a variety of social and cultural differences between groups that give a boundary concrete significance (so that those on one side think of those on the other, "they are not like us because …"). When we discuss ethnicity, the kinds of boundaries we invoke have the character that Weber attributed to the ethnic group: namely, that they are rooted in a "subjective belief in common descent"-i.e., in a shared history based on a common point of origin in the