Can light be localized?

The problem with papers that have a question mark in their title is that the question posed in the title is usually not answered in the paper. This chapter will not be an exception I will review in this chapter some simple concepts that are relevant to the discussion of localization of light. Quantities that will be discussed are speeds of propagation, mean free path and the diffusion constant. Another crucial quantity is the density of states. Finally I will point out some important problems encountered in the theory of light localization.