Studying the interstellar medium of the Milky Way Galaxy gives us the opportunity of identifying in detail the various components of the medium. The equivalent components in distant galaxies may be unresolved, but contribute to the overall emission. We show in Chapter 6 how to deal with emission from unresolved regions. In this chapter we consider the various distinct types of region in the Milky Way that can be explored through molecular line absorptions and emissions. We show that the chemistry in each of these molecular regions is dominated by one or more of the chemical drivers discussed in Chapter 3. The sensitivity of the chemistry to particular physical parameters, discussed in Chapter 4, may be an important concern in some cases. For most molecular regions, we identify a well-known example of each type, which is not necessarily typical but is one in which the consequences of the chemical driver are prominently displayed. We also list some molecular tracers useful in describing the physical conditions in these different situations. We emphasise in particular the tracers of density and temperature for Milky Way conditions. The aim of this chapter is to show how tracer molecules can reveal the nature, origin, and evolution of many types of region in the Milky Way. Tracers of conditions in galaxies external to the Milky Way are considered in Chapter 6.