The rapid rise in obesity in toddlers and young preschool children is a major concern for public health globally. Understanding risk factors for obesity in the early years is therefore fundamental to guiding parents, educators, and health care professionals looking after young children and to developing preventative strategies. Most research has focused on biological risk factors, which can be broadly categorised as genetic predisposition, poor diet (and the behaviours that influence excessive food intake), insufficient physical activity, and the role of developmental factors in early life that influence long-term health. The latter includes establishment of dietary habits and dietary patterns in young children and the impact of a high protein intake on increasing the risk of later obesity. Other risk factors particularly relevant to young children included inadequate sleep, high consumption of sugar-sweetened drinks and large portion sizes of foods servings. Understanding the causes of obesity in preschool children is particularly important in view of the long-term detrimental consequences of obesity in this age group such as greater risk of adult obesity and cardio-metabolic disease. The present chapter reviews the possible causes of obesity in preschool children and its consequences for long-term health, focusing particularly on modifiable nutritional risk factors.