During development we transition from co-regulation (where regulatory processes are shared between child and caregiver) to self-regulation. Most early co-regulatory interactions aim to manage fluctuations in the infant’s arousal and alertness; but over time, co-regulatory processes become progressively elaborated to encompass other functions such as socio-communicative development, attention and executive control. The fundamental aim of co-regulation is to help maintain an optimal ‘critical state’ between hypo- and hyper-activity. Early co-regulatory processes involve both passive entrainment, through which a child’s state entrains to the caregiver’s, and active contingent responsiveness, through which the caregiver changes their behaviour in response to behaviours from the child. Contingent responsiveness operates via negative feedback, through which the behavioural changes from the caregiver compensate for increases and decreases in the child, to help maintain an intermediate critical state. Similar principles, of interactive but asymmetric contingency, drive joint attention and the maintenance of epistemic states as well as arousal/alertness, emotion regulation, and socio-communicative development. Here, we review work that directly observed child-adult co-regulatory interactions in the context of psychopathology. Early co-regulatory processes are thought to play a role in the development of attachment and can develop atypically in a range of ways, across conditions including premature birth, Autism, Attention Deficit Hyperactivity Disorder, anxiety and depression. The most well-known of these is insufficient contingent responsiveness, leading to reduced synchrony, which has been shown across a range of modalities in different disorders, and which is the target of most current interventions. We also present evidence that excessive contingent responsiveness/synchrony can develop in some circumstances. And we show that positive feedback interactions can develop, which are contingent but mutually amplificatory child-caregiver interactions that drive the child further from their critical state. We discuss implications of these findings for future intervention research, and directions for future work.