  This paper deals with the theory of collisions between two ultracold
particles with a special focus on molecules. It describes the general features
of the scattering theory of two particles with internal structure, using a
time-independent quantum formalism. It starts from the Schr\"odinger equation
and introduces the experimental observables such as the differential or
integral cross sections, and rate coefficients. Using a partial-wave expansion
of the scattering wavefunction, the radial motion of the collision is described
through a linear system of coupled equations, which is solved numerically.
Using a matching procedure of the scattering wavefunction with its asymptotic
form, the observables such as cross sections and rate coefficients are obtained
from the extraction of the reactance, scattering and transition matrices. The
example of the collision of two dipolar molecules in the presence of an
electric field is presented, showing how dipolar interactions and collisions
can be controlled.
