The issue of sustainability has been frequently discussed in recent years. Knowledge and its management represent aspects that fit nicely with one of the underlying conditions of sustainability, i.e. durability. Indeed, proper knowledge management can help firms in coping with present and future business challenges. The outsourcing of secondary business functions has become popular with many firms. What is increasingly present is the outsourcing of knowledge competences that is the firms’ core competences. This trend means a challenge for both the firms’ knowledge management and their sustainability. Against this background, the purpose of this paper is to review extant research on the outsourcing of knowledge competences to establish our body of knowledge and to identify gaps justifying further research activities. The study is based on a systematic review of peer reviewed empirical articles on the outsourcing of knowledge competences. This proceeding proves evidence that there are a small number of papers addressing outsourcing and its consequences for knowledge management (KM) sustainability. The recommendations derived from the findings can assist researchers, managers and consultants to better understand the link between knowledge competence outsourcing and KM sustainability. This increased understanding can particularly be useful for managers as better decisions will be possible.