This chapter sets out semantic and analytic parameters for understanding evidentials—closed grammatical sets whose main meaning is information source. A noun phrase may have its own evidentiality specification, different from that of a verb. Other means of expressing information source offer open-ended options in terms of their semantics, and can be more flexible in their scope. Evidentiality is distinct from tense, aspect, modality, mirativity, and egophoricity. An evidential can be questioned or be within the scope of negation. The concept of evidentiality is different from the lay person’s notion of ‘evidence’. Evidentiality involves numerous semantic parameters and cannot be reduced to a simplistic ‘direct’ versus ‘indirect’ opposition. Evidentiality needs to be worked out inductively, based on painstaking work with primary materials on a language, rather than on translation and elicitation. Guidelines for fieldworkers investigating evidentials are offered in the Appendix, alongside a glossary of terms.