The EmDrive is a controversial device first proposed in 2001, purported by its inventors to be a reactionless drive. While no mechanism for operation was proposed, this would violate the law of conservation of momentum and other laws of physics. The concept has at times been referred to as a resonant cavity thruster. The idea is generally considered by physicists to be pseudoscience.

Neither person who claims to have invented it committed to details about it beyond showing prototypes they have built. While the lack of a published design or mechanism makes it hard to say whether a given object is an example of an EmDrive, over the years prototypes based on its public descriptions have been constructed and tested.

In 2016, Harold White's group at NASA observed a small apparent thrust from one such test, however subsequent studies suggested this was a measurement error caused by thermal gradients. In 2018 and 2021, Martin Tajmar's group at the Dresden University of Technology replicated and refuted White's results, observing apparent thrusts similar to those measured by his team, and then made them disappear again when measured using point suspension.

No other published experiment measured apparent thrust greater than the experiment's margin of error. Tajmar's group published three papers in 2021 claiming that all published results showing thrust had been false positives, explaining each by outside forces. They concluded, "Our measurements refute all EmDrive claims by at least 3 orders of magnitude."

Rocket engines operate by expelling propellant, which acts as a reaction mass and which produces thrust per Newton's third law of motion. All designs for electromagnetic propulsion operate on the principle of reaction mass. A hypothetical drive which did not expel propellant in order to produce a reaction force, providing thrust while being a closed system with no external interaction, would be a reactionless drive, violating the conservation of momentum and Newton's third law. Reactionless drives, like other forms of perpetual motion, do not exist in nature, and claims that a drive is reactionless are considered by physicists to be pseudoscience.

The first design of a resonant cavity thruster claiming to be a reactionless drive was by Roger Shawyer in 2001. He called his conical design an "EmDrive", and claimed that it produced thrust in the direction of the base of the cone. Guido Fetta later built a "Cannae Drive", based in part on Shawyer's concept, using a pillbox-shaped cavity.

Since 2008, a few physicists have tested their own models, trying to reproduce the results claimed by Shawyer and Fetta. Juan Yang at Xi'an's Northwestern Polytechnical University (NWPU) was unable to reproducibly measure thrust from their models, over the course of 4 years. In 2016, Harold White's group at NASA's Advanced Propulsion Physics Laboratory reported in the Journal of Propulsion and Power that a test of their own model had observed a small thrust. In late 2016, Yue Chen of the communication satellite division of the China Academy of Space Technology (CAST), said his team had tested prototypes, and would conduct in-orbit tests to determine if they could observe thrust. Martin Tajmar's group at the Dresden University of Technology started testing prototypes in 2015, and by 2021 concluded that observations of thrust were false positives, reporting in the CEAS Space Journal they had refuted all EmDrive claims by "at least 3 orders of magnitude".

Media coverage of experiments using these designs has been polarized. The EmDrive first drew attention, both credulous and dismissive, when New Scientist wrote about it as an "impossible" drive in 2006. Media outlets were later criticized for misleading claims that a resonant cavity thruster had been "validated by NASA" following White's first tentative test reports in 2014. Scientists have continued to note the lack of unbiased coverage.