Group-III nitride (InN, GaN, AlN, and their alloys) heterostructures are promising materials for device applications in optoelectronics and electronics. For accurate design of III-nitride devices, detailed knowledge of their intrinsic electronic properties is important. However, the natural crystalline form of group-III nitrides, the wurtzite structure, induces strong macroscopic polarization (including spontaneous and piezoelectric polarization) fields at the surface and interface. The electrical and optical properties of III-nitride epitaxial films grown in the conventional polar direction are substantially different from those grown in nonpolar directions. This is a systematic study of the relationship between the electronic properties and polarity of group-III nitrides. Chapter 1 introduces the research background for group-III nitrides. Chapter 2 describes related experimental techniques to probe the electronic properties of III-nitrides. Chapter 3 presents studies of the electronic properties of III-nitride surfaces with any crystal orientations, with a special focus on electron accumulation at InN surfaces. The band lineup of group-III nitride heterojunctions is also an important topic for this thesis. Chapter 4 shows the asymmetric valence band offset in III-nitride polar heterojunctions because of spontaneous polarization discontinuities among III-nitrides, explaining why the reported valence band offsets in III-nitrides still show large discrepancies to date. For this reason, our group developed a technique, cross-sectional scanning photoelectron microscopy (XSPEM/S), to achieve the natural band lineup of semiconductors. Using XSPEM/S, the measurement geometry and near-fully relaxed lattice structure allow for the determination of “natural” band alignments without the influence of spontaneous and piezoelectric polarization fields. Chapter 5 presents the XSPEM/S measurements for determining the band lineup of III-nitride. Additionally, this technique (XSPEM/S) can directly visualize the GaN p-n junctions without the influence of crystal polarity and other surface contaminations. Finally, Chapter 6 explains the study’s conclusions and suggests future research topics in group-III nitrides. The realization of group-III nitride semiconductors would be useful for future optoelectronic and electronic device applications.