Massive open online courses (MOOCs) are online courses which typically involve a large number of students with no restriction on enrolment and participation. In many cases, MOOCs focus on academic topics and are offered by universities. MOOCs began to gain popularity in the late 2000s/early 2010s, and were considered to be a possible alternative to the traditional instructional model. Arguably, optimism about MOOCs reached its peak in 2012, when the New York Times named 2012 ‘The Year of the MOOC’ (Pappano 2012). Yet, in 2017, a senior official at Udacity, one of the major MOOC providers, stated that ‘MOOCs are dead’ (Khosla 2017). Given this seemingly dramatic change of opinion on MOOCs, this entry sets out to provide an overview of MOOCs, their history and development, types of MOOCs, research in MOOCs, as well as future opportunities and challenges.