Annex 3 of the German Technical Instruction on Air Quality Control demands for dispersion calculations a Lagrangian particle model in compliance with the German guideline VDI 3945 Part 3. The computer program AUSTAL2000 is a reference implementation of the specifications given in this Appendix. AUSTAL2000 has been developed on behalf of the German Federal Environmental Agency (UBA, UFOPLAN project 200 43 256). The specifications of Appendix 3 themself are already sufficient for a computational implementation of the prescribed algorithms. AUSTAL2000 is provided by the Federal Environmental Agency as a free reference implementation. It can be used both for practical applications and to check other programs that implement Annex 3 of the TA Luft.