What agents do depends not only on what they desire but also on what they believe and what they believe others will do. This makes it possible to influence agents by revealing or withholding information. We will give a short account of the Kripke structure model of knowledge, how it is affected by information exchange and how a knowledge manipulator can influence the actions of other agents by sending signals.
As an aside we mention the role of temperament. What an agent will do in the presence of uncertainty will depend not only on the knowledge and preferences of the agent, but also on the agent's temperament - whether cautious or aggressive. We present some results about how an agent A's knowledge of another agent B's temperament can help A to predict B's choices, and thus allow A to make better choices herself.
One application of knowledge manipulation is campaigning where a politician running for office decides what to say about various issues with a view to affecting the attitudes of the voters towards himself.