The Sponsorship Programme is the central testing programme of the OECD Working Party on Manufactured Nanomaterials (WPMN) launched in 2007. It was called into being to create the conditions for a coordinated approach between the member countries with regard to the safety of manufactured nanomaterials. The WPMN brings together 30 member countries of the OECD and the EU Commission. Some non-member countries are also represented, as are organisations from industry and from standardisation and environmental associations. The idea behind the OECD Sponsorship Programme was to collate data on selected and precisely described manufactured nanomaterials. The intention was thereby to obtain information on their physicochemical properties, behaviour and effects on humans and the environment, and to demonstrate the relationships between them. In this process Germany joined forces with France as joint “Lead Sponsors” for nTiO2. This report summarizes the results of the Sponsorship Programme on ecotoxicology and environmental behavior of nanoscale titanium dioxide of the OECD Working Party on Manufactured Nanomaterials (WPMN).