Using data gathered by way of ethnographic research techniques in an intensive collaborative investigation of the classroom practice of two experienced teachers, each teacher's uniqueness is deliniated, and it is illustrated with examples of classroom practice, what it means for each teacher to know what to do in spontaneous practice. Specifically, the question was asked: In the immediacy of the moment in a classroom, how do teachers know when to change or maintain an approach or method of handling a particular situation? The theoretical perspective of this study derives from the notion of the theory-practice dialectic. The conduct of the inquiry was based on the idea of teachers as theorists and on the belief that understanding of teacher practice can be gained through mutual reconstruction of meaning in action. Teaching is viewed as a professional task; teachers as professional practitioners, owners and users of vast resources of experienced knowledge; and teaching practice as an expression of this knowledge. (JD)