Obesity has been described by the World Health Organisation as a global epidemic, and is recognised as a major health concern due to the associated medical, psychosocial, and economical problems. Treating obesity-related health conditions are a major burden for health systems but treatment towards weight loss are mostly unsuccessful in the long term. Most of the various treatment types are rather costly, like pharmaceutical or surgical interventions. Previous studies found that particular eating behaviours are associated with increased BMI; therefore, an approach of treatment is to influence the concerning eating behaviours towards behaviours with less risk of weight gain. Theories and models of behaviour are used to develop interventions to change eating behaviour. Often treatment schemes focussing on behavioural change are time consuming and costly based on the need of specialised therapists to conduct therapy. Thus, obese and overweight individuals urgently need an effective obesity-management intervention that is simple, inexpensive, and not too time-consuming, does not rely on major assistance of health professionals and can be performed at home for achieving changes in eating behaviour resulting in weight loss. Study aims: The first aim of this current thesis was to develop a new weight loss intervention which focusses on eating behaviour with support of behavioural change theories and current knowledge about eating behaviours which carry obesity risks. The second aim was to investigate the effectiveness of the intervention on body weight and physiological and psychological parameters and to investigate whether particular targeted health behaviours would have a stronger impact on the measured parameters than others. The third aim was to investigate the influence of the various health behaviours on clinical blood parameters which are known to be risk factors for chronic diseases like insulin resistance, type 2 diabetes and cardiovascular disease being related to high BMI. Results: A novel weight-loss intervention was developed, based on principles of Control Theory (CT) and Theory of Planned Behaviour (TPB), using reflective processes on 10 commandments (health behaviours) for the influence of eating behaviour. These 10 behavioural commandments were based on eating behaviours, extracted from literature, which are recognised of high importance for weight loss and health. The study design used three tools (laminated card, leaflet, compliance report). These tools were designed to be complementary to each other regarding their influence on eating behaviour combining the nutritional information with selected behaviour change techniques. Two longitudinal studies to test the effectiveness of the intervention were performed, in United Kingdom (UK) - Bangor area and Saudi Arabia (SA) – King Fahad Medical City. Both studies were performed for 3 and 6 months, as well as including a 3 months follow-up assessment with overweight/obese females. The first study was conducted in UK (3 months, n=35; 6 months, n=36; follow up, n=19) to investigate the effectiveness of the study intervention on body weight, attitudes towards food and food craving domains. The intervention was successful to achieve significant weight loss (5.5 ± 3.7 kg and 7.8 ± 5.3 kg) in the first two phases of the study (after 3- and 6-months of the intervention), respectively. After the follow-up period, the intervention succeeded to maintain the achieved weight loss that was obtained at the end of the second phase (6 months) of the study. There were significant correlations between BMI change and commitment to seven of the eating commandments (B1, B2, B3, B6, B8, B9, B10), as well as after 6-months between commitment to (B1, B2, B3, B5, B7, B8, B9) and BMI change. In the follow-up period, there were significant correlations between BMI change and commitment to B1 and B10. The intervention induced significant improvement in explicit attitudes towards healthy food (3.9 ± 4.5 and 5 ± 7.4) and unhealthy food (4.5 ± 5 and 6.1 ± 6.8) after 3 and 6 months, respectively; outcomes in the follow-up period showed that participants maintained these improvements in explicit attitudes. However, no significant improvement in implicit attitude towards food was detected (IAT). Total of food cravings domains were significantly reduced after 3 months and 6 months of the intervention (33.3 ± 34.3 and 36.8 ± 47.5). These achievements were maintained in the follow-up phase of the study. After 3-months of the intervention, there were significant correlations between weight loss and change of two craving dimensions (Ant+ and Emotions), while BMI change correlated with changes in Ant+ and Hunger. After 6-months, changes of two dimensions of FCQ-t (Ant+ and Hunger) were significantly correlated with weight loss and BMI change. After 3-months, changes in several FCQ-t dimensions were significantly correlated with commitment to some commandments. “Thoughts” dimension correlated with commitment to B1 and B10, and “Guilt” dimension correlated with commitment to B7, while “Intent”, “Cues” and “Hunger” dimensions were correlated with commitment to B1. After 6 months, change in “Thought” dimension was correlated with commitment to B1 and B10, “Intent” dimension correlated with commitment to B1 and B5, “Guilt” correlated with commitment to B4, while “Ant+” and “Hunger” were correlated to commitment with B1. At the follow-up, the reduced level of FCQ-t dimensions remained. The second study was conducted in Saudi Arabia (3 months, n=104; 6 months, n=26; follow up, n=11) at King Fahad Medical City (KFMC) investigating the effectiveness of the novel intervention on body weight loss and important metabolic blood parameters (glucose homoeostasis, lipid profile, blood pressure, and resting heart rate) in overweight/obese females. The intervention was successful to achieve significant weight loss (7.3 ± 3.5 kg, 15.1 ± 8.5 kg, and 2.3 ± 2.6 kg) in the study phases after 3-, 6-months of the intervention, and follow-up, respectively. After 3-months, there was significant correlation between commitment to all commandments and achieved changes in body weight and BMI. However, changes in weight loss and BMI were correlated with commitment to all commandments except commitment to B4 and B6 after 6-months. After 3 months of the intervention there was significant improvement in blood metabolic parameters in terms of glucose homeostasis parameters; significant improvement in fasting insulin, glucose, HG.A1c, HOMA IR, and HOMA S. Improvement of various glucose homeostasis measures were significantly correlated with commitment to all commandments but B4 and B6. Lipid profile significantly improved (reduction in LDL, TC, and TG). Changes in LDL and TC were correlated with all commitment to all commandments but B1 and B10 which have been found to be significantly correlated with HDL change. Systolic blood pressure was found to be reduced after the 3 months of intervention. In terms of attitude measures, explicit attitudes towards healthy and unhealthy food were significantly improved after the intervention; increased attitudes towards healthy food was correlated with commitment to all commandments except B4; decrease in attitude towards unhealthy food was correlated to commitment to all commandments. After 6 months significant improvements in the glucose homeostasis parameters (insulin, glucose, and HG.A1c) and in lipids profile (LDL, TC, and TG) were detected. In addition, there were improvements in explicit attitudes towards healthy foods and unhealthy foods. However, there was no significant change in the cardiovascular parameters (BP and RHR) after 6 months of the intervention. Conclusion: The newly developed weight loss intervention was effective in inducing improvements in terms weight loss, food craving dimensions, explicit attitudes towards food, and achieved significant reductions in blood based clinical health risk parameters. The outcomes support that achievements in weight loss, and the maintenance of weight loss can be achieved using interventional designs based on behavioural theories. As a novelty this has been achieve with an intervention which needs low time commitment from health professionals. The change in body weight via the applied cognitive techniques to alter negative eating behaviours might even be successful in the longterm.