Obesity is considered the second most common health problem in pet cats in developed countries. This PhD thesis was the first to use prospective data from large scale longitudinal UK studies of pet cats ('Bristol Cats' and 'C.L.A.W.s.') to identify early-life risk factors for feline overweight/obesity. Multivariable logistic regression models were used (as no clustering within households was observed). Two factors were significantly associated with owner-reported overweight/obesity at one year of age, assessed using the 5-point Body Condition Score (BCS) system: restricted or no outdoor access (OR, 95% Cl = 1.7, 0.9-2.9) and feeding dry food as the only or major part (> 50%) ofthe diet (1.8, 1.0-3.2)' also measured at approximately one year of age. Six factors were significantly associated with overweight/obesity at two years of age, assessed by owners using images from the 9-point BCS system. These were: being overweight or obese at one year of age (10.6, 4.4 - 25.3); owner belief that BCS 7 was ideal weight, or overweight but they would not be concerned if this were their cat, at questionnaire five completion (33.2,8.5 - 129.4 and 2.7, 1.2 - 6.2 respectively); vets advising owners that the cat should lose weight, or making no comment on their weight, between one and two years of age (12.1, 3.2 - 44.9 and 3.9, 1.5 - 10.3 respectively); owners giving their cat treats when they "felt happy" with them at 18 months of age (2.7, 1.0 - 7.3); feeding ~250g wet food daily between two and six months of age (2.7, 1.2 - 5.9), and feeding dry food as the only or major part (> 50%) of the diet at two years of age (2.1, 1.0 - 4.2). These findings have the potential to reduce the currently high prevalence of a widespread problem, and as such improve the health and welfare of pet cats.