AbstractSocial emotional development in infancy is a predictor of outcomes in later life, yet there is little evidence of effectiveness for parenting interventions designed to enhance social emotional wellbeing in infancy. An 18-month two-arm randomized controlled pilot trial evaluated the feasibility of a definitive trial of Incredible Years (IY) Infant and Toddler parent programs delivered in a proportionate universal model, called Enhancing Social-Emotional Health and Wellbeing in the Early Years (E-SEE) Steps. Intervention families received an IY Babies book (universal dose), followed by the IY Infant and/or the Toddler group-based programs, based on parent depression (PHQ-9) and/or child social emotional development (ASQ:SE-2) scores. Control parents received services as usual. Parents from two English local authorities with a child eight-weeks-old or younger participated, and were block randomized using a web-based system. Primary endpoints for the study were feasibility parameters relating to recruitment, retention, intervention fidelity and appropriateness of measures. 205 participants were randomized (152:53, intervention:control). Our target was 288 parents. Trial retention rate was higher than expected, with a completion rate of 88% (n = 181, 137:44) at follow-up 3; equating to 94% of 192 expected participants. Intervention uptake was lower than expected. Fidelity of delivery was acceptable and measures were deemed appropriate. A definitive trial is feasible with design amendments to include: introduction of a child screener for intervention eligibility; enhanced intervention material; revised sample size and random allocation ratio. Our internal pilot became an external pilot due to these changes.