This note describes the simulation of the CLIC beam delivery system (BDS) using a Geant4 [1] based program BDSIM. BDSIM is designed flexibly in order to track particles through the BDS of a generic linear collider and includes particle interactions and production of secondaries in materials. The code itself is described in Section 2 and this note serves as initial documentation for the programme. The detailed input to the program is controlled by an optics file and cards file, described in Section 10 and Section 11 respectively and the processes present are outlined in Section 4. Results are presented for the tracking performance of the program (Section 3) and its use in determining the backgrounds from halo particles (Section 5); particular emphasis is placed on the determination of muon production rates (Section 6) due to halo loss upstream of the interaction point (IP). Some earlier studies of energy deposition in spoilers are also included for completeness in Section 7. The simulation of a laserwire system is introduced in Section 8
