This paper presents a contribution towards rigourous reasoning about negotiating agents. First, it defines formal models for negotiation and negotiating agents. These models enable to specify the relations between the concepts of plan, plan proposal and resource allocation, on the one hand, and concepts of knowledge, belief and capability, on the other hand. Second, it provides a structured negotiation language enabling to specify primitives, protocols and processes of negotiation. This language is defined by a precise syntax, and it is formally interpreted using a transition system leading to an operational semantics for negotiating agents.