This paper offers a principled framework for the analysis of sources associated with modal verbs that express root necessity. First, the notion of “source” (who or what lies at the origin of the necessity) is described and illustrated, and a comprehensive taxonomy of sources is put forward that can be used for empirical analysis. Special attention is paid to “subject-oriented” sources, which have mainly been discussed in the realm of modal possibility (ability) but which are relevant to modal necessity as well. The framework is then applied to a sample of sentences with Need to extracted from the British National Corpus (BNC). The quantitative study shows that Need to is closely connected with discourse-internal sources, conditional sources, and circumstantial sources. It is not closely associated with a subject-oriented source as it has been defined in the taxonomy and in the context of “rules and regulations.” There are significant differences between the spoken and the written modes in the context of conditional sources and discourse-internal sources.