Romantic relationships hold enormous significance within contemporary society, and monogamous marriage continues to serve as the “master template,” informing the structure, personal and legal parameters of intimacy. Although social changes have indeed impacted traditional notions of marriage and coupledom, monogamy continues to serve as the ultimate embodiment of commitment, love, and devotion to one’s partner and operates as the fundamental framework of sexual and emotional exclusivity. Relationship rules serve as a key indicator of what individuals expect and value within their intimate lives. For example, the rules of marriage (and therefore monogamy) emphasize loyalty, exclusivity, and faithfulness between two partners, which is generally operationalized as fidelity. Further, rule violations are often characterized as “infidelity,” and represent a breach to the commitment established between partners. The rules of monogamy, as well as the consequences for violating them, have been normalized and institutionalized in both paradigm and practice; American culture is decidedly mononormative, and fidelity is central to monogamous relationships. While the master monogamous template continues to be institutionally and individually reinforced, some actively choose to “break the rules” of monogamy in favor of multiple sexual and/or romantic relations. Consensual nonmonogamists, in contexts like open relationships, swinging, and polyamory, challenge the master monogamous template through not only engaging with multiple sexual and/or romantic partners, but also being consensual and usually overt about them. If monogamists have rules about other partners that ensure fidelity, do nonmonogamists have rules? If so, what are they, and what purpose do they serve in a relationship structure that has already broken the cardinal rule of exclusivity? Is commitment important in nonmonogamous relationships, and does fidelity exist between partners who are having sex with and/or falling in love with other partners? This study draws on over 2,000 surveys and 70 in-depth interviews with monogamists, nonmonogamists, and polyamorists to examine the meaning, significance, and practice of fidelity within their intimate relationships. Results indicate that fidelity exists in some variation in all relationship types. The book presents a “Fidelity Typology” based on differentiations between sexual and emotional exclusivity, as well as whether behavior aligns with ideology. The author argues that while exclusivity may not be a necessity in today’s romantic relationships, “feeling special” is key regardless of whether a relationship is monogamous, nonmonogamous, or polyamorous. However, how an individual experiences and ensures specialness is tempered by definitions of love and sex, differentiating between sexual and emotional exclusivity, and engaging individual agency in creating rules between partners. The book highlights that gender and sexual orientation are most salient in conceptualizations of monogamy, sex, and love, rates of nonmonogamy, and even relationship agreements and rules. The author offers a nuanced framework for understanding commitment in today’s romantic relationships, invoking a more agentic approach to achieving specialness called “personal fidelity.” The author argues that while personal fidelity is ultimately socially informed through the master template, it is also largely based on one’s sexual and emotional self-awareness, accountability, and perceived responsibility to other partner(s). Personal fidelity may well be the catalyst for ensuring specialness between partners and preserving the significance of one’s intimate relationship(s). This is an important book for sexualities studies, as well as scholars and students interested in gender, family and intimate relationships.