Electron holography in the transmission electron microscope can provide energy band profiles with sub-nanometer spatial resolution. The phase of the electron beam is sensitive to the electrostatic potential, and a direct measurement of the latter can be achieved by making the electron beam signal that traverses the specimen interfere with a reference electron beam that travels through vacuum. This technique has been quite useful in probing the fields and charges at dislocations and at interfaces in semiconductors. This article presents a review of work done with the participation of the author in the past decade in the use of this technique to determine the piezoelectric effects in group III nitride semiconductor heterostructures. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.