This book describes extensions of deontic logic. Deontic logic is a branch of philosophical logic involving reasoning with norms, obligations, prohibitions and permissions. The extensions concern the logical structure of legal rules and legal reasoning. Their function is to improve the representation of legal knowledge and enhance deontic logic through increased expressibility. The resulting formulas acquire new meanings, not expressible in standard deontic logic, which are subject to fresh interpretations. The author offers an extensive analysis of the representation of actors, to whom the norms are directed, and authorities who enact the norms. Moreover, a distinction is made between enactment and applicability. A modality of enactment can be used to express inconsistent enacted norms in a consistent way. An authority-hierarchy is introduced to filter out the applicable norms from the set of enacted norms. Some related philosophical questions will be discussed regarding the applications of formalisms that are intrinsic to practical science with respect to `consistency' and `universality'. The formalisms and applications considered here are relevant for law, philosophy and computer science, with a special focus on the improvement of legal expert systems and intelligent support for legal professionals.