Though every person has relationships that affect us our entire lives, the process of forming an attachment in adulthood is still largely unknown. Taking the two identified and well-documented phases of relationships in adulthood, romantic infatuation and adult attachment, this dissertation investigates the transition from the attachment-in-the-making infatuation phase to a fully-fledged attachment relationship with a romantic partner. First, a theoretical argument is presented presenting a speculative hypothesis of how this transition happens neurochemically. Through interactions with oxytocin and endocannabinoid receptors and heterodimers, the reward system is shifted to favor familiar and satisfying reward over novel and exciting rewards. Because of this, the reward of attachment is maintained indefinitely and encoded in mu-opiate receptor activation, while infatuation reward, driven by dopamine, ultimately fades over time. This results in the observed phenomenology of each of these phases across individuals, time, and culture. Based on this theoretical model of neurological change between infatuation and attachment, a study was conducted to identify a possible biological marker of attachment formation through pupillary reactions to partner mental representations. Because norepinephrine, identified as high during infatuation and low during attachment, can affect the contraction of iris muscles, it is hypothesized that bringing the partner to mind would cause pupil dilation during infatuation and pupil constriction during attachment. While there was little constriction, there was significantly greater pupil dilation during infatuation than attachment, as well as before many relationship milestones including relationship length. It is possible that this could be used as an unconscious marker of infatuation, and subsequently attachment formation, in adults. Finally, in order to better understand the phenomenology and timing of this transition, a large survey was distributed to examine current and past relationships. Using a classification analysis, the time between eighteen months and thirty months was identified as a time of transition, which aligns with earlier research. However, these data point to the time not being a clean transition, but a gradual one where aspects of both infatuation and attachment are simultaneously experienced. Thus, this dissertation provides a foundation for building further theoretical and empirical work investigating the transition into adult romantic attachment.