This paper presents models in which one agent must decide whether to trust another, whose motives are uncertain. Reliability can only be communicated through actions. In this context, it pays for people to build a reputation based on reliable behaviour; someone becomes credible by consistently providing accurate and valuable information or by performing useful services. The theory provides a justification for long-term arrangements without binding contracts. It also describes those situations where it pays an agent to cash in on his reputation. Many decisions depend on trusting someone. Consumers must decide whether to believe advertisements, banks need to judge the reliability of loan applicants, and employers must decide how much responsibility to delegate to their employees. This paper attempts to model situations that arise when someone is uncertain about whether to trust the people he deals with. In this context, reputations for reliability are valuable. An agent becomes credible by consistently providing accurate, valuable information or by always acting responsibly. I analyse a series of simple, abstract models of information transmission. In the basic model, there are two agents, the Sender (S) and the Receiver (R) who are to play the game a known, finite number of times. At each stage both players learn the value of a parameter measuring the importance of that period's play of the game (that is, the value of making a correct decision) and S obtains payoff-relevant information; S then sends a signal to R, who makes a decision that affects the welfare of both players. After both players learn the consequences of R's decision, the process repeats. At each stage R must decide what action to take. In order to do that he must assess the credibility of S. The difficulty arises because R is uncertain about S's preferences when play begins. I model this by assuming that with positive probability S has identical preferences to R (S is a friend) and with positive probability S has completely opposed preferences to R (S is an enemy). This uncertainty creates a trade-off for enemy Senders. At each stage, providing honest information enhances S's reputation, but only at the expense of foregoing an opportunity for immediate gain by duping R. The possibility that S is a friend lends credibility to what he says, but this credibility may also be exploited by an enemy S. The principal equilibrium has the following form. In the basic model, where S has no control over the importance of a given play, S typically conveys accurate information for the first several periods. R raises the probability he places on S being friendly after receiving accurate information if he believes that an enemy would have lied to him with positive probability. The more important the information, the more likely an enemy will attempt to deceive R. An enemy will eventually take advantage of R by misleading him. When this happens, the Sender loses all opportunities for deception in the future. I