Worldwide, childhood obesity is a significant public health challenge. Similar to rates in other developed countries, one in four three-year olds in Ireland is overweight/obese, with increased rates amongst those from lower socio-economic backgrounds. A number of risk factors, identifiable during the antenatal period and infancy, are associated with the development of overweight/obesity. While the number of interventions delivered during this timeframe is increasing, results are mixed, with limited impact shown on weight-related outcomes measures. Furthermore, it is not known which components of these determine effectiveness, as no studies to date have focused on the discrete behaviour change techniques (BCTs) used. It is therefore difficult for practitioners, researchers and/or intervention developers to understand the most important and effective, transferable, intervention components. Health professionals can play an important role in preventing childhood obesity in part due to the large number of routine contacts they have with parents. This paper will present the findings of a systematic review of the evidence for the effectiveness of health professional-delivered interventions to promote obesity prevention in children under the age of two, identified through systematic searches (including CINAHL, Embase, MEDLINE, PsycINFO, PubMed, Open Grey, Scopus, and Web of Science), and the behaviour change theories and/or techniques associated with intervention outcomes. Intervention procedures are coded using the BCT taxonomy while the methodological quality of studies is assessed using the Cochrane Collaboration tool for assessing the risk of bias. By better understanding what works (or not), extant early life obesity prevention interventions can be optimised/re-developed.