In this communication crystallographic and chemical inhomogeneities occurring in GaN single crystals, homoepitaxial layers and quasi-bulk thick epitaxial layers are described. Practical classification of defects as (i) growth-related and (ii) processing-induced is given. On the basis of numerous studies using different examination techniques (DIC and DF optical microscopy, SEM) and methods of revealing (defect-selective etching, PEC etching, X-ray diffraction and TEM), the most dangerous technology steps for the formation of defects are described and the types of defects, which might be critical for performance/operation of the GaN-based opto-electronic devices are indicated. It is shown, that the routine use of a simple defect-selective etching method during the subsequent stages of the technology of GaN homo-epitaxial lasers is helpful in achieving time-effective device processing. (c) 2005 Elsevier B.V. All rights reserved.