This paper presents a decision theoretic framework that an autonomous agent can use to bid effectively across multiple, similtaneous auctions. Specifically our framework enables an agent to make rational decisions about purchasing multiple goods from a series of auctions that operate different protocols (we deal with the English, Dutch, First-Price Sealed Bid and Vickrey cases). The framework is then used to characterise the optimal decision that an agent should take. Finally, we develop a practical algorithm that provides a heuristic approximation to this ideal.