"Interpersonal relationships are a central element of human existence. While cultures vary with respect to their individual versus collectivistic orientation (Triandis, 2018), even in the most individualistic of cultures people are to some extent embedded in networks of relationships. Individual experience is situated within a context that involves some combination of family, friends, community, and society. We feel the presence - or for some the absence - of these social forces. That does not mean relationships are uniformly sources of tremendous joy - they can also inspire indifference or become sources of pain. I mean to suggest only that the bonds we form to other people, or the absence of such bonds, are powerful influences on our behavior. To the extent that I am inspired by something in psychology, it is the idea that we move each other around as we move through the world. I am fascinated by the impact interpersonal relationships have on personal behavior. I have been exceedingly lucky in this sense. I arrived at my early career as a clinical psychologist just in time to see something of a "relationship renaissance" in research on HIV prevention and treatment. What vision I had for my career was reshaped by the 2009 publication of Sullivan and colleagues' influential paper indicating that as many as 68% of new HIV infections among sexual minority men (a group that includes gay, bisexual and other men who have sex with men) in the United States were transmitted between main or primary relationship partners. Reading it was the first time I recall being really excited about what I could potentially do as a behavioral scientist. This book represents the culmination of just over a decade of work that followed that realization"--