Traumatic Brain Injury (TBI) is one of the most common causes of death and disability in children and adults under 35 years of age, and is a major health concern worldwide (Anderson, Catroppa, Morse, Haritou, & Rosenfeld, 2001; Babikian & Asarnow, 2009; Feigin, et al., 2013; Langlois, 2005). It is estimated 10 million people are effected worldwide annually (Hyder, Wunderlich, Puvanachandra, Gururaj, & Kobusingye, 2007), and it is projected that TBI will be the third largest cause of global disease burden by 2020 (The Lancet Neurology, 2010). Symptoms of TBI can persist across cognitive, behavioural, emotional, and social domains, regardless of age; pre-school children being particularly vulnerable, due to rapid development of the brain within the first years of life. The main purpose of this study (aim 1) was to determine whether mild traumatic brain injury (mTBI) injury in pre-school children had an effect on their normal cognitive development, specifically executive functioning (EF) and the sub domains of inhibitory control and working memory 24 months post injury compared with an age-matched control group. Behavioural functioning was also examined in terms of its relationship to EF (aim 2), specifically addressing both internalising and externalising behaviours and adaptability. Lastly factors that may contribute or be predictive of risk of injury were explored (aim 3), specifically age/maturation of the brain or stage of development, and both parental/environmental factors. Forty four children participated in this population-based study; twenty two were pre-schoolers aged two years of age at the date of injury, and were assessed 24 months post injury and were aged between 4 years – 4 year 11 months. This group was matched by age, gender, and geographical region with a TBI free control group (n=22). Both the parent-rated Behaviour Rating Inventory of Executive Functioning (BRIEF-P) and the performance based Colour Object Interference and Day & Night tasks (COI & D&N) measures of executive functioning were used to determine any differences between the groups in terms of inhibitory control and working memory. Further parent-rated measures the Behavioural Assessment System for Children (BASC-2) and a screening test (SDQ) were also used to measure behaviour and social and emotional functioning, specifically internalising and externalising behaviour and adaptability. Other parental and environmental measures were used to determine parental mental health; The Hospital Anxiety and Depression scale (HADS), the home environment; Home Observation for Measurement of the Environment (HOME) and the parent child relationship using a parent child interaction observation task (PCI). The study found that pre-schoolers with mTBI exhibited higher levels of internalising, and externalising problem behaviours, and poorer adaptability and EF skills, as evidenced by the associated correlations. However analyses revealed there were no significant differences between the mTBI and control groups in terms of the measures of EF, behaviour and social and emotional functioning. These findings could suggest assessing EF in children aged between 2-5 years is difficult, or that the manifestations of the outcomes of these impairments are not yet fully observable due to young age. The limitations of the study were the relatively small sample which was not sufficient enough to carry out regression analyses; the comparison group was not representative of the general population and lastly, the measure of the HOME was found to be out-dated and as such did not provide a comprehensive picture of the home environment. Future studies could aim to more closely examine the relationship between age at injury and outcomes from mTBI. Also they could utilise the categories within the mild severity category for a more sensitive account of the consequences of mild traumatic brain injury in children. Therefore more research needs to be done on the effects of mild traumatic brain injury rather than focusing on hospital cohorts.