Cocaring encompasses how parents and teachers work together to coordinate caregiving. Two critical components of cocaring have demonstrated importance for child and parent well-being: support and undermining. Although parent-teacher relationships have been studied via qualitative interviews and through self-administered questionnaires, little observational research has been conducted. This study sought to develop an observational approach to measure the quality of cocaring relationships by recording and coding parent-teacher interactions. Eighteen mother-teacher dyads of 6- to 36-month-old children from six childcare centers were video recorded during morning drop-off and evening pick-up times. Fifty-eight videos were coded for eight dimensions of cocaring quality, representing observed support and undermining. Results indicated strong inter-rater reliability and associations amongst the individual dimensions consistent with theory, thus supporting initial construct validity. This study offers a new observational tool to assess cocaring relationships and provides practical information about the kinds of behaviors that convey support and undermining in everyday interactions.