This is a thesis that deals with the norms and rules of players playing online games together. It is also a thesis about believability, the current capabilities of non-player characters (NPCs) and the attitudes amongst game developers towards dynamic and systemic games AI. The primary theme of this thesis considers which means of communication and coordination in terms of norms and rules are present in groups of players and particularly in guilds and clans playing Massively Multi Online Games (MMOGs) and First Person Shooters (FPSs) respectively. The presence of norms in these types of groups has been overlooked in previous research even though guilds have been addressed to some extent. When rules have been discussed in games research, the actual use, meaning and interpretation behind these rules from a player perspective has been omitted. In this thesis rules and norms are interpreted from a guild and clan perspective as important means for coordination, used in order to keep the group together. The implicit rules are further seen as implicit rules made explicit through guild and clan forums where these groups of players express how to preserve the shared game experience. The absence of rituals, norms and rules has also been studied in temporary groups of one MMOG, with the explanation that existing relations with other players are maintained in these game sessions, but new relations are usually seen as too costly to invest in. The second theme is directed at believability and the state of current NPCs, how immersion is influenced by NPCs that do not act in believable ways. The second theme is also influenced by the first theme, whereby rules and norms are seen as valuable tools for creating believability in NPCs, directly targeting the social layer, a slightly overlooked area of research. The last section is directed at applying the results from the first section, how players play by the rules and norms of the group, and how this could foster believability in NPCs. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 5: In press. Paper 6: In press.</p>