The terahertz (THz) portion of the electromagnetic frequency spectrum—approximately ranging from 0.3 THz to 3 THz—is recently gaining significant attention. Numerous applications, such as security screening and high data-rate wireless links to name two relevant examples, can benefit the non-ionizing character of THz radiation with respect to X-rays and the increased resolution and bandwidth with respect to microwave techniques. It is essential to accurately characterize the THz properties of materials with appropriate techniques capable of dealing with dielectric, metal-dielectric, and conductive samples and provide results over a relatively wide range of frequencies with sufficient frequency resolution. In this respect, THz time-domain spectroscopy (THz-TDS) is a valuable tool, although the commonly used transmission mode fails to be effective when used on highly reflective samples, such as thin conducting films or metasurfaces, which are both components that are widely used in THz applications. On the other hand, the less explored THz-TDS in reflection mode has recently been proven to be a very effective technique for accurately characterizing such kinds of components, as well as most of the dielectric and conductive materials used in THz applications. For this purpose, in this abstract, we will show the measurement protocols that we recently developed for obtaining an accurate electromagnetic characterization of materials and components at THz. Specific care will be paid to the dispersive models employed to simultaneously fit the THz-TDS measurements and comply with electromagnetic theory. We will show a variety of experimental validations over commercial dielectric materials (e.g., Rogers, ciclo-olephins, and dry resists), novel 2-D materials (e.g., graphene, graphene oxide, and transition metal dichalcogenides), thin conducting films (e.g., aluminum zinc oxide, indium tin oxide, and titanium), and metasurfaces (e.g., patch arrays, strip gratings, and fishnets). Finally, we will briefly comment on how the latter components can profitably be used to realize THz radiation devices.