This is joint work with Bai-Ling Wang. The purpose is to establish a surgery formula for the Seiberg-Witten Floer homology. Given an embedded knot in a homology 3-sphere Y, there is an exact sequence connecting the Seiberg-Witten Floer complexes of the homology 3-sphere Y₁, obtained as +1 surgery on the knot, of the original homology sphere Y, and of the 3-manifold obtained by zero-surgery on the knot. At the level of generators, the result is obtained as a decomposition of the moduli space of solutions of the Seiberg-Witten equations according to the cutting and pasting of the underlying three-manifold. An analysis of the spectral flow leads to a choice of consistent grading of the Floer complexes. In order to compare the boundary operators, it is necessary to understand the behaviour of the flow lines under cutting and pasting. Finally, the results of this analysis can be used to prove the existence of the exact sequence.