Towards a Loop-Tree Duality at Two Loops and Beyond

We present an extension of the duality theorem, previously defined by S. Catani et at on the one-loop level, to higher loop orders. The duality theorem provides a relation between loop integrals and tree-level phase-space integrals. Here, the one-loop relation is rederived in a way which is more suitable for its extension to higher loop orders. This is shown in detail by considering the two-loop N-leg master diagram and by a short discussion of the four master diagrams at three loops, in this sketching the general structure of the duality theorem at even higher loop orders.