The purpose of the article is to study the functional peculiarities of language units of uncertainty, to present their classification and to understand the reasons that lead to their widespread use in various types of discourses. The classification of language units of uncertainty is based on their functional features. The possible list of their functions includes the following features: they act as a link between the old and the new information; they serve as a reference to an open list of elements that is difficult to enumerate to the end; they can act as markers of general uncertainty, when the subject of the conversation does not require precise wording; when speakers focus on important information, disregarding details; when the speaker uses hedging techniques and some others. Our study proved that markers of the category of uncertainty are not considered as redundant, and are viewed as qualifying elements that structure the content of the statement. The results of the study, as well as the presented ill