This review on selective oxidations is split into two parts. The first part concerns catalytic gas-phase oxidation reactions in micro-reactors, typically being performed in wall-coated micro-channels [V. Hessel, G. Kolb, J.C. Schouten, V. Cominos, C. Hofmann, H. Löwe, G. Nikolaidis, R. Zapf, A. Ziogas, E.R. Delsman, M.H.J.M. de Croon, O. de la Iglesia, R. Mallada, J. Santamaria, in: S. Ernst, E. Gallei, J.A. Lercher, Rossini, E. Santacesaria (Eds.), Conference pre-prints of the DGMK/SCI-Conference "Oxidation and Functionalization: Classical and Alternative Routes and Sources", Milan, Italy, October 12–14, 2005  (see also acknowledgements at the end of the article)]. Liquid and gas–liquid oxidations are not included due to their different reactor design and way of processing. This comprises process development for fine chemical intermediates or bulk chemicals. By example of different reactions, the benefits of micro-chemical process engineering are shown. While there are numerous engineering reasons, one major driver is the increase of selectivity by diminishing side and follow-up reactions, most often the total oxidation to carbon dioxide, through preventing or at least decreasing hot spot formation. Another major advantage of micro-reactors is that mass-transfer resistances can be suppressed, thereby giving access to intrinsic kinetics. In the second part, we describe selective oxidation as one gas purification step in the framework of fuel processing for fuel cells, which is most often termed preferential oxidation in this context. Here, the selectivity towards carbon monoxide formation by diminishing the hydrogen oxidation is a major driver. The current developments are grouped so that all facets from kinetic modelling, heat transfer studies, catalyst testing, reactor and integrated-system engineering up to process engineering, exergy analysis, performance benchmarking and operation under real-case process flows are covered.