It is a currently popular view that essences, laws, and logic are among the grounds of necessity and possibility. Another such idea relates essences, laws, and logic closely to causal explanations and to non-causal grounding explanations. The Sources of Necessity combines those tenets within the view that certain phenomena establish explanations because they exert necessity, a modal force, on the facts. In form of a slogan, the explanatory phenomena are the sources of necessity. The book investigates this idea, and it argues that essences, laws, and logic are the sources of necessity. The investigation contains two different strands, one pertaining to explanation and one to necessity. In the first strand, The Sources of Necessity uses different ways of exerting necessity to analyse the explanatory behaviour of essences, laws, and logic. It investigates the governance of natural laws, the interplay of essences and logic in the generation of absolute necessity, and the existence and nature of metaphysical laws. The second strand concerns the nature of necessity. It is argued that necessity is primitive and that there are several equally fundamental species of necessity. Two particularly thorny questions about necessity that arise from these insights are, ''Why are the sources of necessity themselves necessary?'', and ''Why are different kinds of necessity ordered by strength?'' The book develops a rationalist view to answer those questions. It says that contingent reality is the result of a cosmogenic sequence that unfolds from essences and logic through the other sources all the way to the contingent facts.