"Based on key term searches and forward and backward citation mapping, we constructed a review of 223 studies published since 1972 related to countermeasures designed to combat influence operations. Each identified study included: (1) a source of variation in exposure to countermeasures; (2) a clearly defined outcome of interest for some specified population; (3) relevance to thinking about the potential of an intervention to impact real-world behavior; and (4) enough detail to evaluate the credibility of the findings. This approach amounts to sampling the foundational research surrounding countermeasures and thus incorporates the collective judgement of this emerging field. All of the studies we identified examined user-focused countermeasures, i.e., those aimed at the consumers of disinformation. None looked at countermeasures aimed at impacting the influence operations directly. There exists a mismatch between the major interventions taken by platforms - algorithmic downranking, content moderation, redirection, and deplatforming accounts - and those studied by the research community. Most papers we reviewed focus on one particular method for countering information operations: fact-checking and its many offshoots. The types of interventions employed by social media companies on actual users are understudied. We recommend further research on four key areas: (1) measuring the impact of the most common interventions by social media platforms, (2) assessing the impact of countermeasures on real-world behaviors (both online and offline), (3) evaluating the efficacy of countermeasures in non-Western contexts, and (4) studying countermeasures that target the creators of disinformation content in addition to studying consumer-facing policies." (Essay summary)