The purpose of this article is to summarize eight potential threats to internal validity that occur with randomized field trial (RFT) studies. Depending on specific contextual factors, RFTs do not necessarily result in strong internal validity. Of particular concern is whether the unit of random assignment is the same as the number of replications of the intervention, threats as a result of local history, and subject effects. The eight threats are described, with suggestions for providing adequate monitoring to know if they rise to the level of likely or plausible threat to internal validity. Accessed 19,617 times on https://pareonline.net from December 03, 2007 to December 31, 2019. For downloads from January 1, 2020 forward, please click on the PlumX Metrics link to the right.