In this paper, we review some of the main methods to characterize on-state and off-state losses in wide-band-gap devices under switching conditions. In the off-state, we will discuss about losses related to charging and discharging the output capacitance in wide-band-gap devices, both in hard- and soft-switching. In the on-state, we will present an accurate measurement of dynamic on-resistance degradation, particularly in Gallium Nitride (GaN) devices. These losses are typically not described in data-sheets, but can be a dominant loss mechanism in power electronic applications.