Asynchronous Transfer Mode ( ATM ) networks, which are the emerging standards for the future broadband-ISDN networks, allows for the input tra c from users to vary both from one call to another and within the call. ATM speci es a method for controlling the tra c ow across the user network interface ( UNI ). This involves each user negotiating call parameters with the network. Once these parameters have been decided then a contract is made between the user and network. The network must then enforce the contract in order to guarantee performance and quality of service to other users. What is of interest to the network is given a particular set of users and contracts what is the worst tra c that the users could input to the network while still abiding by their contracts. This type of input tra c would be called the worst case tra c as it would produce the lowest performance in the network. The reason why this is of importance is that for simulating network performance we w ould like t o h a ve the worst case tra c inputs. Furthermore, it's important for the tra c controller, network, to know the possible worst case tra c so that it can assign parameters accordingly. The contract parameters have been decided upon by the standards organisations and what is needed now is to decide what type of tra c can pass these tests and produce the lowest network performance. This problem has been studied in the literature. We g i v e some theoretical background to explain some of the results in the litrature, and we further look at some examples of types of worst case tra c sources. We show that for the two most common types, the greedy on{o and the three state source that either can be worse, depending on the situation. This would imply that there is no general worst case tra c as has been considered for the last number of years.