After defining happiness or subjective well-being, some explanatory theories or models of happiness and factors forming the basis of this construct are presented. Personality variables seem to be much more important than sociodemographic and contextual ones. Perhaps the best model is one which consists of dynamic and interactive equilibrium between personality and other variables, between endogeneous and exogeneous factors. Finally it is argued that happiness is a complex and multidimentional construct, difficult to understand and assess. Therefore, it is concluded that only a holistic perspective can give a full account of happiness.