The present dissertation focus on the neurophysiological correlates underlying interactive behavior, considering that social behavior has been accounted as a core dimension involved in developmental outcomes and, thus with important implications for the establishment of therapeutic relationship. Therefore, this dissertation addresses two main research topics: 1) the neurophysiological signatures associated with interactive behavior in 1-month-old infants; and 2) the physiological correlates underlying the therapeutic dyad interactive exchanges, within the therapy context. Regarding the neurodevelopmental studies, visual and auditory stimuli intensities were offered to 1-month-old infants, while their central and peripheral nervous system activity was recorded. Additionally, the infants were assessed, in regards to their social behavior, with a neurobehavioral scale. Results showed that higher visual evoked-potentials (VEP) and auditory evoked-potentials (AEP) P2 amplitude to the higher intensity stimuli. VEP N3 amplitude was found to be positively associated with adjusted orienting and state regulation behaviors, but in the in the lower intensity condition. Similarly, greater P2 amplitude in the lower intensity was positively correlated with the same behaviors. Moreover, P2 amplitude in the lower intensity was found to predict language abilities in 12-month-old infants. Furthermore, results showed that young infants displayed an increase in heart rate to the higher auditory stimuli intensity. In addition, vagal tone, to both auditory intensities, was found to be positively associated with regulatory abilities in 1-month-old infants, which are essential for social involvement. These results suggest that the neurophysiological correlates are observed in regards to different sensory stimuli intensities processing. Moreover, they seem to be associated with adjusted social behavior and developmental competences, evidencing that early in the development we can identify specific neurophysiological markers underlying adjusted interactive behavior. The identification of these markers may contribute to an early identification of developmental-related problems. Considering the studies focused on the therapeutic relationship, specifically, we aimed to characterize the collaboration process, and the underlying physiological correlates, occurring in the initial phase of a good outcome case. The therapeutic dyad was assessed in regards to their physiological reactivity, during each therapeutic session. Both therapist and client heart rate (HR) activity was recorded. Afterwards, therapy session’s collaboration process was characterized by coding the different therapeutic exchanges according to the Therapeutic Collaboration Coding System (TCCS). In order to verify if HR pattern between the dyad was associated with the collaboration process, physiological concordance and discordance was calculated for each therapeutic episodes identified. Results showed that, collaboration process in the first session is mainly characterized by collaborative exchanges, where therapist’s interventions that are coded as supporting problem and client’s response as safety. Therapist’s challenging interventions occurring in this session were commonly invalidated, producing non-collaborative episodes. As the initial phase of the therapy moves on, novelty interventions introduced by the therapist tend to be more frequently validated. By the forth session, the episodes now occurring are more often challenging, from the therapist’s side, followed by safety or tolerable risk responses, from the client’s side (producing collaborative episodes). In general, collaborative episodes, specifically supporting problem-safety, were accompanied by physiological concordance, as therapist and client’s HR is similar. In non-collaborative episodes, commonly invalidated challenges, the client tends to increase his HR, which is not accompanied by the therapist’s HR and, therefore, physiological discordance is observed. Nevertheless, in collaborative episodes that are characterized as validated challenging interventions, both therapist and client’s HR increased, which may be associated with the client’s perceiving and accepting new perspectives. This acceptance is followed by physiological signature (i.e. increase HR), which seems to be associated with cognitive and emotional processes. These results suggest that specific collaboration exchanges are happen in the initial phase of a good outcome case and, furthermore, such exchanges seems to be accompanied by different HR activity patterns that are associated with collaborative and non-collaborative episodes. Overall, this dissertation presents evidence that specific neurophysiological correlates are associated with interactive behavior involved in infancy development that may offer a framework to understand the dynamics of the physiological processes regarding interactive behaviors in the therapeutic relationship.