An increasingly popular class of Networked Virtual Environments is Massively Multiplayer Online Games (MMOGs), where massive amounts of players interact simultaneously in virtual game worlds. As the demand increases for MMOG systems that support millions of players, so does the need for solutions to their scalability problems. The servers, and in some cases also clients, are required to work properly under heavy load, whilst maintaining an accurate game state, minimizing latency, and ensuring that bandwidth requirements do not exceed a maximum. Although many scalability oriented solutions exist, few comprehensive surveys are available. In this article, we describe a taxonomy of scalability of MMOGs, and map surveyed solutions onto its classes. We find that many promising solutions are available, but insufficient research has been done on combinations of them. Therefore, in search of the best performing system, we recommend to combine promising solutions into a couple of different systems, to test each system’s performance.