  A model calculation for pion production in nucleon-nucleon collisions is
presented. Direct production, pion rescattering and contributions from pair
diagrams are taken into account. The amplitudes for the elementary processes
are based on well established microscopic models of nucleon-nucleon and
pion-nucleon scattering. The $\Delta (1232)$ is included explicitly and is
found to play an important role even at energies close to production threshold.
A good overall agreement with existing data from the pion production threshold
up to the Delta resonance region is achieved.
