The unique or enhanced properties of manufactured nanomaterials (MNs) suggest that their use in nano-enabled products will continue to increase. This will result in increased potential for human and environmental exposure to MNs, during manufacturing, use, and disposal of nano-enabled products. Scientifically based risk assessment for MNs necessitates development of reproducible, standardized hazard testing methods such as those provided by the Organization of Economic Cooperation and Development (OECD). Currently, there is no comprehensive guidance on how to best address testing issues specific to MN particulate, fibrous, or colloidal properties. This paper summarizes the findings from an expert workshop convened to develop a guidance document that addresses the difficulties encountered when testing MNs using aquatic and sediment OECD test guidelines. Critical components were identified by workshop participants that require specific guidance for MN testing: preparation of dispersions, dose metrics, the importance and challenges associated with maintaining and monitoring exposure levels, and the need for reliable methods to quantify MNs in complex media. To facilitate a scientific advance in the consistency of nanoecotoxicology test results, we identify and discuss critical considerations where expert consensus recommendations were and were not achieved, and provide specific research recommendations to resolve issues for which consensus was not reached. This process will enable development of prescriptive testing guidance for MNs. Critically, we highlight the need to quantify and properly interpret and express exposure during the bioassays used to determine hazard values.