In this thesis, the main focus is on deontic logic as a tool for formal representation of moral reasoning in natural language. The simple standard system of deontic logic (SDL), i.e. the minimal Kripkean modal logic extended with the deontic axiom, stating that necessity (interpreted as obligation) implies possibility (interpreted as permission), has often been considered inadequate for this aim, due to different problems, e.g. the so-called deontic paradoxes. A general survey of deontic logic and the problems with SDL is made in chapter 1. In chapter 2, a system denoted Classical Deontic-Modal logic (CDM1) is defined. In this system, there is a primary obligation operator indexed to sets of possible worlds, and a secondary requirement operator, defined in terms of strictly necessary conditions for fulfilling an obligation. This secondary operator has most of the properties of the necessity operator in SDL. In chapters 3 and 4, it is argued that CDM1 is able to handle the SDL problems presented in chapter 1 in an adequate way, and the treatment of these problems in CDM1 is also compared with their treatment in some other well-known deontic systems. In chapter 5, it is argued that even though the problems related to quantification in modal contexts are relevant to deontic logic, these issues are not specific to deontic logic. In chapter 6, the relations between some controversial features of moral reasoning, such as moral dilemmas and “non-standard” deontic categories like supererogation, and deontic logic are discussed. It is shown how CDM1 can be modified in order to accommodate these features.