Cyberbullying is an incipient phenomenon which occurs by means of digital devices, in virtual environments, which often overlaps with traditional bullying. Research reveals the relevant role played by bystanders in stopping bullying and cyberbullying. The aim of this work is to identify those factors which encourage or hamper the mobilisation of young bystanders under 20 years of age in instances of cyberbullying, through a systematic literature review spanning 2005 to 2016 in the databases Web of Science Core Collection and SciELO Citation Index. A total of 19 articles were analysed. We identified two types of factors. Firstly, there are contextual factors, which refer to the relationships at play, the interactions and the environment, and are grouped into the following categories: friendship, social environment, bystander effect, incident severity, action of other bystanders, request for assistance, evaluation of the situation, knowledge of effective strategies, characteristics of virtual environments, and fear of retaliation. Secondly, there are personal factors, referring to individual traits, categorised into: empathy, moral disengagement, self-efficacy, behavioural determinants, previous experience of bullying and cyberbullying, and demographic and socio-economic data. Of particular influence seem to be the factors of friendship and social context, as well as empathy, moral disengagement and self-efficacy. To formulate practical recommendations to guide the development of educational programmes aimed at preventing cyberbullying using the bystander approach, further evidence is needed in relation to all factors, although certain general directions can be discerned even at this early stage. This is a new and exciting field of research which carries the hope of eradicating bullying in all its forms.