In 2007, a genome wide association study identified a SNP in intron 1 of FTO with increased BMI. Homozygous risk allele carriers are on average three kg heavier than those homozygous for the protective allele. Mouse models have been made, including a conditional knockout, which is lean when globally expressed, as well as a conditional overexpression allele, which has increased body weight when globally expressed. The results from these and other studies suggest that the FTO SNPs lead to weight gain by increasing FTO activity and/or expression. Adult inactivation of Fto using the tamoxifen inducible Cre demonstrated that removal of Fto may be as deleterious as overexpression, with the adult knockout mice having increased fat mass and decreased lean mass. It also supported the role FTO plays in development as adult inactivation of Fto did not increase mortality rates as seen in the global Fto-/- pups. This study also revealed the importance of effective energy expenditure analysis in the mouse. I have confirmed a link between Fto-/- and increased mortality, which may be caused by alterations to developmental processes. Fto-/- reduces cilia formation in MEFs and results in dysregulated cilia formation in specific tissues in Fto-/- embryos. Levels of FTO also appear to affect adipogenic differentiation, which could be due to altered WNT/β-CATENIN signalling. Pharmacological inhibition of FTO was a success in vitro and a compound screen identified FG2216, which could be used in vivo to inhibit FTO. The in vivo effects of FG2216 at 60 mg/kg/2days did not affect body weight or composition in the mouse. My research suggests that there is dysregulation of gut hormones and neuronal signalling pathways in the FTO overexpression mice, which could cause the hyperphagia and increased body weight. These studies add to our current knowledge of FTO function, and suggest a role for FTO in control of body composition, development, and satiety signalling.