Travaux préparatoires (preparatory works) are the records of a treaty's development & negotiation. Use them to help understand the evolution of a treaty. Many U.N. treaties include the travaux préparatoires on the individual treaty's website or the treaty body's website:
e.g.: United Nations Convention against Transnational Organized Crime
Preparatory documents may also be found within the documents & meeting records for the conference or negotiating body. Try some of the tools listed on the Documents & Publications page of this guide.
Use some of the resources suggested below or search Omni for commentaries & guides to travaux préparatoires. Search by treaty name.