The uncertainty reduction theory (URT), also known as initial interaction theory, developed in 1975 by Charles Berger and Richard Calabrese, is a communication theory from the post-positivist tradition.

It is one of the few communication theories that specifically looks into the initial interaction between people prior to the actual communication process. Uncertainty reduction theory originators' main goal when constructing it was to explain how communication is used to reduce uncertainty between strangers during a first interaction. Berger explains uncertainty reduction theory as an "increased knowledge of what kind of person another is, which provides an improved forecast of how a future interaction will turn out". Uncertainty reduction theory claims that everyone activates two processes in order to reduce uncertainty. The first being a proactive process, which focuses on what someone might do. The second being a retroactive process, which focuses on how people understand what another does or says. This theory's main claim is that people must receive information about another party in order to reduce their uncertainty and, that people want to do so. While uncertainty reduction theory claims that communication will lead to reduced uncertainty, it is important to note that this is not always the case. Dr. Dale E. Brashers of the University of Illinois argues that in some scenarios, more communication may lead to greater uncertainty.

Berger and Calabrese explain the connection between their central concept of uncertainty and seven key variables of relationship development with a series of axioms and deduce a series of theorems accordingly. Within the theory two types of uncertainty are identified: cognitive uncertainty and behavioral uncertainty. There are three types of strategies which people may use to seek information about someone: passive, active, and interactive. Furthermore, the initial interaction of strangers can be broken down into individual stages—the entry stage, the personal stage, and the exit stage. According to the theory, people find uncertainty in interpersonal relationships unpleasant and are motivated to reduce it through interpersonal communication.

The foundation of the uncertainty reduction theory stems from the information theory, originated by Claude E. Shannon and Warren Weaver. Shannon and Weaver suggests, when people interact initially, uncertainties exist especially when the probability for alternatives in a situation is high and the probability of them occurring is equally high. They assume uncertainty is reduced when the amount of alternatives is limited and/or the alternatives chosen tend to be repetitive.

In 1975, Berger and Calabrese created uncertainty reduction theory "to explain how communication is used to reduce uncertainties between strangers engaging in their first conversation together". Previous researchers had approached interpersonal communication from empirical perspectives. Hypotheses had been derived from social psychological theories as well. However, the lack of focus on interpersonal communication process motivated Berger and Calabrese to form hypotheses that directly involve communication behavior.

Further research expanded uncertainty reduction theory's explanatory power in areas such as verbal communication, nonverbal communication, intimacy, reciprocity, and information seeking, etc. Scholars suggest that people should use a "variety of sources when collecting information to reduce their uncertainty." The variety of sources should include the words being expressed (verbal communication) as well as the physical mannerisms coupled with those expressions (nonverbal communication).

Brashers helped to further expand the scope of uncertainty reduction theory outside the traditional dyadic interpersonal exchange. Now, when scholars talk about uncertainty reduction, they tend to take humanity's discomfort with uncertainty as a basic tenet of what it means to be human.

There are seven assumptions associated with the uncertainty reduction theory: