Oxidative coupling of methane (OCM) is considered one of the most promising routes to directly convert methane into more valuable hydrocarbons. The uncertain economics related to the tradeoff between conversion and C-2 selectivities is the primary reason why OCM is currently not industrially applied. In the last decades, numerous studies have focused on developing a viable catalyst that has the potential to improve the low C-2 yields. But is the primary issue of OCM truly a catalyst problem? Because of the high exothermicity of the OCM process, thermal effects and path dependence are dominating in all OCM reactors of practical importance. Furthermore, irreducible diffusion limitations exist on the pellet scale. Understanding how to exploit these mass and heat transfer effects by reactor engineering is a prerequisite for the breakthrough of OCM. In this work an overview is given of criteria used to assess mass and heat transfer resistances, parametric sensitivity and runaway in catalytic packed bed reactors. The importance of mass transport limitations, runaway and/or ignition for OCM, either on the pellet scale or on the reactor scale, is shown in several examples. Both simple and advanced reactor concepts are discussed with a focus on their heat and mass transfer characteristics. Clear progress has been made in the past lustrum on all these fronts and it seems that research is on the verge of a real breakthrough to make OCM happen on an industrial scale. (C) 2018 Elsevier Ltd. All rights reserved.