The fuzzy theory was introduced by Professor Zadeh (Inf Control 8(3):338–353, 1965) for the first time. This theory is useful for modelling the vague and uncertain systems and made a way for introducing other theories of the literature such as uncertainty theory, etc. for modelling uncertain systems. According to Prof. Zadeh, the membership of the fuzzy subsets is inexact and many of the sets in real-world have such property. Since 1965 that this theorem was introduced, many mathematical structures and methods have been introduced for dealing with uncertain systems that more or less are based on the fuzzy theory. In this section, some definitions and theorems about fuzzy theory are reviewed, then the fuzzy numbers and their ranking functions are introduced. In continue, the presented contents are developed for type-2 fuzzy sets and numbers. An application of transportation problem is also provided for this chapter.