Heterogeneous networks (HetNets) are considered a key enabling technology to provide high capacity for next generation, also known as fifth generation (5G), networks. However, in order to efficiently exploit the advantages of HetNets and strive towards high network efficiency, efficient user equipment (UE) association is decisive for targeting network performance goals. Therefore, in this work, user association algorithms are surveyed and classified based on the criterion they use for association with the base stations (macrocells or small cells). Special focus is given on the suitability of the user association algorithms for 5G as well as their complexity.