Obesity can be expected to be among the major health burdens of the 21st century. Within the USA, the prevalence of obesity (defined as having a BMI >25 kg/m2) has increased to almost 65% of the adults. The case numbers in Europe, Australia and affluent countries of Asia are far from different. Unfortunately, behavioural and dietetic interventions have proven to be ineffective concerning their long-term success in the most cases and medical interventions are not only expensive, but are also compromitted by side-effects. Thus, the attitude of many physicians towards obese people is frequently characterised by frustration and accusation, since, up to now, the most cases of obesity are regarded as the result of overnutrition, sedentary lifestyle and lack of physical activity in general. However, our conscience has evolved, that things may be not always be that easy to explain. During the last years, several new molecules playing important roles in lipid metabolism and energy homeostasis were identified and have profoundly enhanced our understanding of obesity. Furthermore, the identification of monogenetic forms of obesity, sometimes causing the lack of one single substance involved in lipid metabolism as e.g. leptin or receptor deficiencies for these molecules have helped us in understanding some of the mechanisms resulting in an obese phenotype. In this book, the authors try to characterise rare forms of obesity with overeating genetically determined, or forms of obesity not caused by overnutrition at all. Apart from the description of the rare forms of obesity, the book also encompasses new insights into the strengths and weaknesses of dietary intervention, approaches to prevent obesity, further reasons of obesity and the regulation or dysregulation of appetite behaviour. All articles represent the state of the art and will hopefully help the reader to gain profound insights into the rapidly expanding field of obesity research.