The presence of lattice point defects at non-equilibrium concentrations and of non-doping impurities can never be completely prevented in the growth and processing of semiconductors, and such imperfections can have serious effects, often deleterious but sometimes beneficial, on the electrical properties of the semiconductor and on the electronic devices manufactured therefrom. It is therefore essential to be able to detect and understand those imperfections. Appropriate electrical measurements on semiconductors - conductivity & Hall coefficient in homogeneous structures, and current-voltage, thermally-stimulated capacitance & current, photo-capacitance & deep level transient spectroscopy (including under applied uniaxial stress) in diode structures - as described in detail with up-to-date examples in this paper, lead not only to knowledge and understanding of the imperfections themselves, but also provide information on the properties of the imperfections that affect the electronic and opto-electronic behaviour of the semiconductor.