Contents - 8[-]Preface - 12[-]1.[-]String theory - 14[-]2.[-]Pure spinor formalism - 58[-]3.[-]Basic techniques - 76[-]4. BRST quantisation of the[-]pure spinor superstring - 98[-]5. Decoupling of unphysical[-]states - 118[-]6.[-]Discussion and conclusion - 156[-]Appendix A - 160[-]Bibliography - 168[-]Samenvatting - 174[-]Acknowledgments - 180String Theory