Terahertz time-domain spectroscopy in reflection mode has been recently proven to be an effective technique for the nondestructive, contactless characterization of the electromagnetic properties of materials. In this work, we provide an overview of the methods that we have developed so far for characterizing the generally complex permittivity and conductivity of bulk and two-dimensional materials. Experimental results are reported for two considerably different examples such as the retrieval of the complex permittivity of a silica substrate and the sheet resistance of a thin titanium layer on a cyclo-olefin substrate.