Background: There are several standards which make explicit a consensus view on sound practice in systematic reviews (SRs) for the medical sciences. Until now, no equivalent standard has been published for SRs which focus on human health risks posed by exposure to environmental challenges, chemical or otherwise. Objectives: To develop an expert, cross-sector consensus view on a key set of recommended practices which can function as a standard for the planning and conduct of SRs in the environmental health sciences. Methods: A draft set of practices was derived from two existing standards for SRs in biomedicine and discussed at an international workshop of 33 participants from government, industry, non-government organisations, and academia. The guidance was revised over six follow-up webinars, multiple rounds of email feedback, and bilateral phone calls, until there was group consensus that a comprehensive framework for the planning and conduct of high-quality environmental health SRs had been articulated. Results: The Conduct of Systematic Reviews in Toxicology and Environmental Health Research (COSTER) standard is a code of practice consisting of 70 performance elements across eight performance domains, representing the consensus view of a diverse group of experts as to what constitutes “sound and good” practice in the conduct of environmental health SRs. Discussion: COSTER provides a set of practices which, if followed, should facilitate the production of credible, high-value SRs of environmental health evidence. COSTER clarifies sound and good practice in a number of controversial aspects of SR conduct, including the management of conflicts of interest, inclusion of grey literature, and protocol registration and publication. Not all of the practices are yet commonplace but environmental health SRs would benefit from their use.