‘Selective catalytic reduction’ (SCR) has emerged as the most efficient technology to curtail ‘nitrogen oxide’ (NOx) emissions from diesel-powered vehicles and power plants. In this process, the selective reductants of ammonia or urea react with oxygen and harmful NOx to produce harmless nitrogen gas and water. Vanadia-based catalysts were the first materials and have been commercialized for SCR applications since the 1970s. Their modest cost and superior sulfur resistance have promoted their widespread use, particularly in stationary SCR installations. On the other hand, metal-exchanged zeolites have started to prevail in recent years for mobile SCR applications. Whereas copper-exchanged zeolites offer unrivaled low-temperature activity and a wide operating window, analogous iron-based materials have the advantage of negligible formation of undesirable nitrous oxide at comparably low costs. Nowadays, the advancement of SCR is inextricably linked to modern spectroscopy and mechanistic studies under operating conditions. They provide structure–reactivity relationships that help to facilitate a rational catalyst design and synthesis. Against the backdrop of worsening air quality and tightening environmental legislation, the advancement of the SCR technology continues to remain of utmost importance.