Four general topics which require further research are discussed in this chapter. The first of these is the problem of the 'supply side' in location models, with particular emphasis on residential location and with the inclusion of supply - demand reconciliation as a part of the problem. The second topic is the problem of equilibrium structures and solution techniques. This includes both intermodel and intramodel equilibria. The third topic is the calibration problem. Here, too, there are questions of calibration both within and between models. The fourth topic is the broad, and crucial, one of the interplay between theoretical model building and the availability of data with which the models can be tested.