EM, Em or em may mean:

• Effective Microorganisms or EM-Technology by Dr. Teruo Higa .
• Electromagnetism (diamagnetism) as compared to ferromagnetism (paramagnetism).
• An abbreviation for the Electron microscope.
• An EM (major chord) or Em (minor chord) used in musical notation for string instruments or 'Chordophones'.
• The em dash (—) is defined as one Em in width in orthography or typography.
• Em space is one Em wide. A term used for orthography or typography.
• An Enlisted Man of the United States Army.
• One Exametre equals 10¹⁸ metres or 1 E18 m.
• Expectation-maximization (EM) algorithm is an algorithm used for statistical computing.
• A phoneme of the Cyrillic alphabet letter 'M'. It corresponds directly to the Latin alphabet letter 'M' and is pronounced similarly.
• Also one of the Spivak pronouns.
• A common slang for "them". Ex: Go get'em tiger!