This chapter develops a simplified discrete-time version of Kremer's model (Kremer QJE 1993) aimed at explaining the existence of a transition from a stagnation regime (where GDP per capita remains constant despite continuous technological progress) to the modern growth regime (where GDP per capita grows continuously despite population growth). As such, this chapter provides a first illustration of what a Unified Growth Model can bring to the study of the long period. We also use that model to cast some light on the Industrial Revolution, and its particular timing and location in space.