The nitride semiconductors are widely used in high efficiency light emitting devices and are currently being considered for photovoltaic applications. The reduced symmetry in the wurtzite structure compared to cubic semiconductors results in the growth of large densities of crystalline defects and in the presence of strong spontaneous and piezoelectric polarization effects. A correlation between the microstructure and the polarization fields can be achieved with electron holography in the transmission electron microscope. Electron holograms thus obtained 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, and it is particularly useful to determine the piezoelectric effects in group III nitride semiconductor heterostructures. A review of applications to InGaN and AlGaN based heterostructures is presented in this paper.