By means of regulatory cooperation, ttip is to increase the depth of cooperation between the eu and the usa on regulatory issues. This article sketches out the envisioned regulatory cooperation as far as this is possible based on the eu negotiating texts and assesses it from an environmental perspective. In this perspective particular attention need to be paid to differences in the stringency of regulation of sectors of environmental relevance (e.g. food, chemicals and cosmetics) in the us and the eu. As a result, adverse environmental impacts can only be avoided if harmonization takes place at a high level. Sub-statutory regulations and private sector standardization such as iso standards may also be of considerable environmental relevance. The example of manipulated vehicle emissions of nitrogen oxides shows how important even the apparently technical issue of effective verification of exhaust emissions can be for environmental protection. The criteria used to assess risks and deal with uncertainties are highly relevant. The precautionary principle embodied in German and European law can be found neither in us nor in international trade law. In should remain a part of ttip (and more prominently so) and not be sacrificed in last minute negotiations.