Modality is a particularly interesting area developmentally, as it is concerned with the child’s evaluation of the possibilities and obligations involved in everyday interpersonal encounters. This paper will present the findings of a longitudinal case study of one child’s development of modality over a 21 month period, beginning at 2;6 (two years and six months) and extending until 4;3 years. The study analyses the child’s use of epistemic and deontic modal auxiliaries, as well as other devices for expressing modality such as projection and modal adjuncts (Perkins 1983, Halliday 1994). Data were recorded in the child’s home during spontaneous interactions. The study identifies the origins and functions of modal expressions in early childhood, including the development of interpersonal grammatical metaphor (Halliday 1994), sometimes referred to as conversational functions (Shatz, Wellman and Silber 1983, Bartsch and Wellman 1995). The move from subjective to objective orientations and the use of adjectives and adverbs to convey modal meanings are discussed. The study demonstrates an inextricable link between the content or area of learning (the child’s interest in the mental processes of herself and her younger sister) and the modal expressions which provide a vehicle for this learning.