A relatively new field, called interpersonal neurobiology, draws its vigor from one of the great discoveries of our era: that the brain is constantly rewiring itself based on daily life. In the end, what we pay the most attention to defines us. How you choose to spend the irreplaceable hours of your life literally transforms you. All relationships change the brain - but most important are the intimate bonds that foster or fail us, altering the delicate circuits that shape memories, emotions and that ultimate souvenir, the self. Brain scans show synchrony between the brains of mother and child; but what they can't show is the internal bond that belongs to neither alone, a fusion in which the self feels so permeable it doesn't matter whose body is whose. Wordlessly, relying on the heart's semaphores, the mother says all an infant needs to hear, communicating through eyes, face and voice. Thanks to advances in neuroimaging, we now have evidence that a baby's first attachments imprint its brain. The patterns of a lifetime's behaviors, thoughts, self-regard and choice of sweethearts all begin in this crucible. As a wealth of imaging studies highlight, the neural alchemy continues throughout life as we mature and forge friendships, dabble in affairs, succumb to romantic love, choose a soul mate. Loving relationships alter the brain the most significantly.