To ensure that the tests used to address the safety of Manufactured Nanomaterials are consistent and defensible; the OECD launched the Sponsorship Programme for the Testing of Manufactured Nanomaterials (Testing Programme) in November 2007. This Testing Programme verifies the testing methods used on Manufactured Nanomaterials by pooling the expertise of OECD member countries, some non-member countries and other stakeholders to fund the safety testing of specific Manufactured Nanomaterials. This document is the summary of the dossier on Multiwalled carbon nanotubes (MWCNT). The Summary includes information on physical and chemical properties; general information on exposure; hazards to the environment; and toxicological information.