In the master’s thesis we study finite rings and their groups of units. The invertible elements of an arbitrary ring with a multiplicative identity form a group for the corresponding multiplicative operation. In the special case of the ring Z_n, this is a group denoted by U_n, also called the group of units. The main purpose of the thesis is to introduce the ring Z_n and to determine, for each integer n, the well known group that the group of units U_n is isomorphic to. We also determine the necessary and suficient condition on n for the group U_n to be cyclic. To this end, we investigate the structure of the group of units U_n and show that we can write it as a direct product of certain cyclic groups. We also indicate how our results can be used for solving some congruence equations.