The present work is a review of the topological theory of defects and inter
faces in crystalline materials, aimed at illustrating cases that have been
studied under this framework in nitride films and interfaces. The review ad
resses crystallographic calculations in hexagonal and trigonal crystals usi
ng the Frank system. It also presents the principal methods of a priori and
a posteriori defect characterization in a unified manner, showing their eq
uivalence, the circumstances under which they are applicable, and practical
aspects of their deployment. A number of experimental observations are ana
lysed, and useful conclusions are extracted, for example, concerning admiss
ible defects, structure-mechanism relations, and the influence of defects o
n properties. These include (i) threading, stacking-fault and interfacial d
islocations in GaN epilayers, (ii) the influence of the epitaxial interface
on the systematic appearance of inversion and stacking disorder, (iii) str
uctural transformations of inversion domain boundaries due to their interse
ctions with stacking faults, and (iv) double-positioning twinning in TiN co
ntact layers deposited on GaN.