In media studies, mass communication, media psychology, communication theory, political communication and sociology, media influence and the media effect are topics relating to mass media and media culture's effects on individuals' or audiences' thoughts, attitudes, and behaviors. Through written, televised, or spoken channels, mass media reach large audiences. Mass media's role in shaping modern culture is a central issue for the study of culture.

Media influence is the actual force exerted by a media message, resulting in either a change or reinforcement in audience or individual beliefs. Whether a media message has an effect on any of its audience members is contingent on many factors, including audience demographics and psychological characteristics. These effects can be positive or negative, abrupt or gradual, short-term or long-lasting. Not all effects result in change; some media messages reinforce an existing belief. Researchers examine an audience after media exposure for changes in cognition, belief systems, and attitudes, as well as emotional, physiological and behavioral effects.

The influences of mass media (or 'media effects') are observed in various aspects of human life, from voting behaviors to perceptions of violence, from evaluations of scientists to our understanding of others' opinions. The overall influence of mass media has changed drastically over the years, and will continue to do so as the media itself develops. In the new media environment, we have dual identities - consumers and creators. We not only obtain information through new media, but also disseminate information to wide audiences.

Further, the influence of the media on the psychosocial development of children is profound. Thus, it is important for physicians to discuss with parents their child's exposure to media and to provide guidance on age-appropriate use of any media, including television, radio, music, video games and the Internet.

There are several scholarly studies which addresses media and its effects. Bryant and Zillmann defined media effects as "the social, cultural, and psychological impact of communicating via the mass media". Perse stated that media effects researchers study "how to control, enhance, or mitigate the impact of the mass media on individuals and society". Lang stated media effects researchers study "what types of content, in what type of medium, affect which people, in what situations". McLuhan points out in his media ecology theory that "The medium is the message."

Media effects studies have undergone several phases, called media effects paradigms, often corresponding to the development of mass media technologies.

During the early 20th century, developing mass media technologies, such as radio and film, were credited with an almost irresistible power to mold an audience's beliefs, cognition, and behaviors according to the communicators' will. The basic assumption of strong media effects theory was that audiences were passive and homogeneous. This assumption was not based on empirical evidence but instead on assumptions of human nature. There were two main explanations for this perception of mass media effects. First, mass broadcasting technologies were acquiring a widespread audience, even among average households. People were astonished by the speed of information dissemination, which may have clouded audience perception of any media effects. Secondly, propaganda techniques were implemented during war time by several governments as a powerful tool for uniting their people. This propaganda exemplified strong-effect communication. Early media effects research often focused on the power of this propaganda (e.g., Lasswell, 1927). Combing through the technological and social environment, early media effects theories stated that the mass media were all-powerful.

Representative theories: