The socio-emotional experiences of infants during transitions to early childhood education and care (ECEC) and across their first year in these out-of-home contexts are not well known. In an international project across five countries (New Zealand, Finland, Australia, Scotland and the United States), observational data, video of key moments, plus re-probing interviews with parents and teachers concerning 10 infants (six females) aged 5-13 months were collected across the first year of ECEC. An embedded case study design was used to analyse infant experiences from both quantitative and qualitative perspectives. Findings indicated low positive affect on infants' first day of ECEC that increased after the first week and throughout the first year of ECEC. Drop-off periods remained a time of negative emotionality for the first month of ECEC. Over time more positive emotional experiences were evident, with peer interactions fostering positive affect, activeness and involvement, while one-on-one interactions with teachers were an important context for interpersonal engagement and well-being. These findings shed light on the highly charged emotional experience for infants when they transition to institutional care, and how these might be ameliorated in practice.