In recent decades, human and animal studies have made strides in advancing knowledge about the neural and genetic basis of attachment—the selective preference for specific individuals, which is characterized by proximity-seeking, care, and the investment of time and resources. Pair-bonding is one instantiation of attachment, but it is unique for its role in reproduction (with genetic implications) and biparental care of offspring. Unique to humans, and sometimes coinciding with pair-bonding, is the complex phenomenon of romantic love, a basic human motivation involving an intense desire to be united with the beloved physically, emotionally, and mentally. Increasingly, scientists have examined the biological basis of romantic love, providing a deeper understanding of the mechanisms underlying attachment, pair-bonding, and its nature. In this chapter, we provide an overview of animal models detailing the biological mechanisms of pair-bonding. Next, we review research on the neural and genetic basis of pair-bonding and romantic love in humans. Finally, we comment on future directions of this work and elucidate how the neural and genetic processes discussed herein influence the expression and function of this basic human drive that we call love.