Existence of risk is directly connected with uncertainty.
Uncertainty assumes availability of factors at which results of actions are not determined, and extent of potential impact of these factors on results is unknown; it is incompleteness or inaccuracy of information on project sales terms.
Distinguish the following three types of a situation:
 definiteness situation when the choice of the specific action plan from a set always leads possible to the known, precisely certain outcome;
 a risk situation at which the choice of the specific action plan, generally speaking, can lead to any outcome from their fixed set. However for each alternative probabilities of implementation of a possible outcome are known, i.e. each alternative is characterized final by a probabilistic set;
 the uncertainty is characterized by the fact that the choice of a concrete conduct can lead to any outcome from the fixed set of outcomes, but probabilities of their implementation are unknown. Here it is possible to allocate two cases: or probabilities are unknown owing to lack of necessary statistical information, or in general it does not make to tell sense about objective probabilities.
Thus a risk situation is characterized by the following signs:
 uncertainty availability;
 need of the choice of alternatives of actions (at the same time it must be kept in mind that the refusal of the choice is also a kind of the choice);
 an opportunity to estimate the probability of implementation of the chosen alternative since in an uncertainty the probability of approach of events in principle is not established.
The risk situation is a kind of an uncertainty when approach of events is probable and can be certain. In other words, the risk is the probability estimated in any manner, and uncertainty is what does not give in to assessment.
Speaking about uncertainty, we will note that it can be shown differently:
 in the form of probabilistic distributions (distribution of a random variable is precisely known, but it is unknown what specifically value will accept a random variable);
 in the form of subjective probabilities (distribution of a random variable is unknown, but the probabilities of separate events determined in the expert way are known);
 in the form of interval uncertainty (distribution of a random variable is unknown, but it is known that it can accept any value in a certain interval).
