This lecture is about building origin-destination (OD) matrices and OD matrix estimation. After placing this topic within the general framework of this course, it is explained how to determine choice functions. The explanation is concluded with an example. Thereafter, trip generation and trip distribution functions are discussed. The mathematical background of the latter is elaborated and an example using Poisson distribution is given. As an alternative to the Poisson model, Hyman’s method is discussed and an example is given. The second part of the lecture is about the estimation of OD matrices. Finally, special topics are discussed.