The authors say their book, "covering a period of more than two millennia, does not pretend to be all-encompassing." But it comes pretty close. This sweeping survey of women's friendships from biblical times to the present (way more than two millennia!) is breathless in the best sense of the word, touching just long enough on a succession of fascinating, well-summarized examples to draw out the common threads from Naomi and Ruth to the present day. What struck me most powerfully in the first half is how much more difficult women's history would be to write if not for women's close, intensely loyal friendships with each other. For instance, we learn that the remarkable poetry of the Mexican Sor Juana Ines de la Cruz would have remained unknown had her friend Maria Luisa de Laguna not brought it to the attention of the Spanish court. And there's Madame Roland's memoirs of the French Revolution, which we have only because her friend Sophie Grandchamp smuggled them out of prison. The same goes for sheer survival: "The Social Sex" contains countless stories of women who supported female friends who were destitute or in crisis.Women's friendships have been important from the earliest times because, with our male-dominated economic structure, women are more more likely to need to rely on each other than are men. Yalom and Brown don't gloss over the important differences of time, place, social class, and race -- in fact, they suggest that overcoming these barriers may be more important now than ever. Yet they underscore some important constants, critically the degree to which their friendships have enabled women (Hildegard of Bingen, the Blue Stockings of 18th century England, Jane Addams and Eleanor Roosevelt and their circles) to educate themselves, grow personally, and carry out ambitious projects that the male power structure would never have let them pursue otherwise. Working class women's friendships were largely invisible for most of this history, but Yalom and Brown touch on it where they can, and when the literature in this area widens out in the 20th century, they give us this story, too. The same goes for friendships between women in non-white communities. And they give us a good account of the changing means of friendship in recent times, including social media and the threat to "traditional" women's friendships in our atomized and commodified cultural landscape. If anything, however, women's friendship is more important today than ever: "With the uncertainties surrounding marriage, it is likely that friendships will continue to offer forms of support women would once have found within their families," Yalom and Brown write. They are right, and this fascinating survey gives us the historical perspective necessary to see why.