In Multi-Agent Systems (MAS) agents cooperate with each other to provide the overall functionality of a MAS. To develop open MAS, new methodologies for Agent Oriented Software Engineering (AOSE) are required, that account for openness. Current AOSE methodologies are only concerned with analysis and design of MAS with a fixed number of agents, a static structure and fixed interactions and relationships between agents. In open MAS harmful agents unknow at design time of a MAS may enter the system. In recent years trust models have been developed to prevent the incorporation or facilitate exclusion of harmful agents. AOSE methodologies currently do not incorporate concepts that facilitate trust modeling. This paper presents the basis of a meta-model for trust and how it can be integrated into existing AOSE methodologies in order to better support modeling of open MAS.