Summarizes electrical measurement data in GaAs materials and devices, and describes in detail the techniques used to obtain these data and the ideas behind them. Special emphasis is given to subjects sometimes ignored in other works such as impurity and defect Fermi functions, degeneracy factors and multiband conduction, and also to relatively new subjects such as the application of magnetoresistance to determine carrier mobility in device structures. Some of the information is quite practical, e.g., how to make ohmic contacts or where to buy a commercial, automated Hall-effect apparatus. Includes many detailed derivations.