This article explores the notion of the evidential category of mutual knowledge. Mutual knowledge mainly refers to knowledge which is jointly constructed via linguistic interaction and shared perceptual experience. Sources of information for mutual knowledge cover the contributions of conversational participants, together with their jointly held beliefs and assumptions. Interlocutors employ individual knowledge evidentials to introduce information and use mutual knowledge evidentials to establish facts by consensus. Once established, this shared knowledge is marked as such in subsequent speech. Evidentials are thus displayed to be part of a system for building up the epistemic base shared between speakers in dynamic, interactive discourse. In South Conchucos Quechua and in Sihuas Quechua the individual and mutual knowledge categories are formally identified via dedicated enclitics in paradigmatic contrast. The article describes and illustrates the five-choice evidential system of South Conchucos and the six-choice system of Sihuas, then compares the two systems with each other and with the more well-known three-choice system of Cusco Quechua, a system in which mutual knowledge forms have not been attested. The comparison of the three evidential systems proposes a sequence of stages in the development of mutual knowledge as a grammatical category. The findings proposed here are mainly based on spontaneous conversation, the setting in which mutual knowledge forms principally reside and in which epistemic authority is carefully negotiated.