Co-regulation (or coregulation) is a term used in psychology. It is defined most broadly as a "continuous unfolding of individual action that is susceptible to being continuously modified by the continuously changing actions of the partner". An important aspect of this idea is that co-regulation cannot be reduced down to the behaviors or experiences of the individuals involved in the interaction. The interaction is a result of each participant repeatedly regulating the behavior of the other. It is a continuous and dynamic process, rather than the exchange of discrete information.

Co-regulation is often applied in the context of emotions. In this sense, the emotions of each individual within a dyad are constantly in flux, depending on the emotions and behaviors of the partner. If emotion co-regulation is in effect, the result will be a decrease in overall emotional distress. A working definition of emotion co-regulation has been offered as "a bidirectional linkage of oscillating emotional channels between partners, which contributes to emotional stability for both partners". Emotion co-regulation is thoroughly studied in the context of early emotional development, often between infants and caregivers. It has also been studied in adult interpersonal interactions, with an emphasis on close, romantic relationships. One important note regarding co-regulation is that there may be an imbalance within the dyad, such that one member is more actively regulating the behavior of the other.

Despite a history of studies on co-regulation, researchers have lacked a clear, operational definition of co-regulation. In a review of emotional co-regulation in close relationships, Butler and Randall proposed the following three criteria for determining the presence of co-regulation:

Co-regulation has been identified as a critical precursor for emotional self-regulation. Infants have instinctive regulatory behaviors, such as gaze redirection, body re-positioning, self-soothing, distraction, problem solving, and venting, but the most effective way for an infant to regulate distress is to seek out help from a caregiver. Sensitive, reliable responses by the caregiver, over time, indicate to the infant that emotional distress is manageable, either with the help of a caregiver, or by strategies developed during past interactions with a caregiver.

The following is an example of co-regulation between a mother and her infant, from Emotional Development: The Organization of Emotional Life in the Early Years. This scenario exemplifies a mother maintaining her infant's engagement via variations in her voice, facial expressions, and body language. She sensitively elicits behaviors from the infant and adds more stimulation when appropriate. Likewise, the infant indicates and maintains the mutual engagement with her own facial expressions, vocalizations, and body language.

Hello there pumpkin ... Mommy's comin' to get you. Yes, she is. [Brief pause.] Momma's gonna get you and tickle you. What do you think of that? [Brief pause.] Come on. Come on, you little sweetie. Let me see that smile. Humm? [Pause.] Yeah, that's right ... thaaaat's right. [The infant exhibits a big smile with bobbing head, and mother responds in kind, then says,] Oh, well now, are you gonna say somethin'? Are ya? [Pause, mother nodding head, widening eyes.] Come on! [Pause.] Come on! [The baby begins cycling movements of the arms and kicking the feet.] Come on. [Drawn out, then longer pause.] Yeah! [as the baby bursts forth with a gurgling sound, the caregiver then laughs and hugs the baby]."

In infancy, co-regulation is primarily initiated and maintained by the caregiver, such that the caregiver is putting forth effort to soothe the child's emotional distress. Caregiver responses are calibrated to closely fit infant responses and elicit the next behavior. This effect has been called "caregiver-guided dyadic regulation". Co-regulatory interactions between parents and children become more balanced over time, as children develop emotion regulation strategies of their own. Caregivers of preschoolers, for example, take a more passive co-regulating role. They demonstrate willingness to assist with distress and availability when needed, but not over-involvement. Instead of more actively co-regulating, these parents are more likely than parents of younger children to give their children space to independently regulate, suggest regulation strategies, or facilitate more abstract discussions regarding emotional experiences and appropriate responses.

The strongest theoretical and empirical support for this phenomenon comes from research on attachment theory. Attachment has even been explicitly defined as "the dyadic regulation of emotion". The basic premise is that early biological and behavioral co-regulation from the caregiver facilitates the child's development of secure attachment which then promotes self-regulation.