Within the grammar of the world’s languages, knowledge can be expressed in various ways. We focus on the grammatical expression of four major groups of meanings related to knowledge: I. Evidentiality: grammatical expression of information source; II. Egophoricity: grammatical expression of access to knowledge; III. Mirativity: grammatical expression of expectation of knowledge; and IV. Epistemic modality: grammatical expression of attitude to knowledge. The four groups of categories interact. Some develop overtones of the others. For instance, some evidential terms may take on egophoric, mirative, or epistemic meanings. Evidentials stand apart from other means of expressing knowledge in their scope, possibility of double marking, time reference different from that of the predicate, the option of being negated or questioned separately from the predicate of the clause, and specific correlations with speech genres and social environment. Evidentials can be semantically complex. They may combine reference to the information sources of the speaker and of the addressee, and access to information source. Evidentials and epistemic modalities display an unequal relationship. Evidentials often arise form reinterpretation of epistemic markers; developments in the opposite direction are restricted. In a situation of language obsolescence, the erstwhile evidentials may undergo reinterpretation as modals, as the obsolescent language succumbs to a dominant one with no evidentials. Evidentials show a number of dependencies with other grammatical categories, including polarity, tense, aspect, person, and number. A few of these dependencies can be explained by the history of the development of evidential distinctions in the language.