Simulation is an important method to evaluate the design of communication systems in general. Unlike conventional mobile wireless networks, packets in Opportunistic Networks are forwarded when nodes-radios move in range of each other, i.e. opportunity to transmit arises. As a result, the performance of forwarding and routing algorithms in opportunistic networks is heavily influenced by the frequency and the duration of physical encounters between nodes. In addition, the time scales involved between these encounter events are typically much larger than for mobile ad-hoc networks. We consider the problem of characterizing encounter networks, defined as a traditional graph representation of communicating nodes, but with the additional time dimension. We set out to model encounter networks from experimental contact traces of mobile phones carried by students during a 6 months long experiment. We then propose EMO, an Encounter-based MOdel designed for opportunistic network application, its evaluation and implementation in a popular mobile network simulation package. http://www.maths.usyd.edu.au/u/AppliedSeminar/abstracts/2008/ardon.html