Coordination between infant and adult is thought to be essential to development. However, evidence on this topic is sparse. The research in this Monograph--grounded in a dyadic systems perspective and relational psychoanalysis--addresses the issue of vocal coordination. The research employs an automated apparatus to examine the micro-second vocal coordinations of 4-month old infants with their mothers and with strangers. These coordinations are then used to predict infant attachment and cognition at 12 months. The results demonstrate that coordination is related to the contexts of partner (mother/stranger), site (home/lab) and outcome (attachment/cognition). Although "more" is often assumed to be "better," a midrange of coordination was found to be optimal for attachment. However, for cognition a high degree of coordination between stranger and infant in the lab was optimal. There was more mutual coordination between infant and stranger than infant and mother. This suggests that mutual, or bi-directional, coordination assesses vigilance rather than "attunement." The coordination between infant and stranger predicted attachment status just as well as that between infant and mother. In addition, infant and stranger coordination was the most powerful cognitive predictor. This work further defines a fundamental dyadic timing matrix that guides infant development.