Summary. The article defines Liouville numbers, originally introduced byJoseph Liouville in 1844 [17] as an example of an object which can be approximated "quite closely" by a sequence of rational numbers. A real number x is a Liouville number iff for every positive integer n, there exist integers p and q such that q > 1 andIt is easy to show that all Liouville numbers are irrational. Liouville constant, which is also defined formally, is the first transcendental (not algebraic) number. It is defined in Section 6 quite generally as the sumfor a finite sequence {a k } k∈N and b ∈ N. Based on this definition, we also introduced the so-called Liouville number as