Even with the current rise in awareness of sexual and intimate diversity, monogamous relationships remain the cultural norm. Most people aspire to it, and the state encourages it, providing legal and financial benefits to married couples; however, statistics show adultery is commonplace, marriage rates are falling, and divorce  figures are rising. This entry in the Big Idea series traces the evolution and normalization of the monogamous ideal—questioning whether it is “natural” or not—and surveys the spectrum of alternative relationship models that people are seeking out in a world of internet dating and birth control. It explores the emotional and psychological facets of ethical polyamorous relationships; questions whether these relationships benefit men disproportionately and whether they are compatible with raising children; and assesses the likelihood that diverse forms of multipartner marriages and large friendship networks will become the norm in the future. With over 150 color images and incisive, engaging, and authoritative text, examines society’s attachment to monogamy, evaluates its benefits and limitations, and assesses the merits of polyamorous relationship models in our modern world.