In this chapter, we present and compare two quite different organisations of teachers’ collaborative work: that of lesson study as a means for professional development of mathematics teachers in Japan, and that of Danish high school teachers’ collaboration in the setting of multidisciplinary modules. Each of these turns out to be crucially affected by certain school level paradidactic infrastructures, defined conditions and constraints of teachers’ collaborative work in preparing, observing and evaluating actual teaching. The systematic and comparative study of paradidactic infrastructures is proposed as a way to interpret and ultimately overcome difficulties which arise, for instance in attempts to realise major educational reforms.