In this thesis algorithms for checking isomorphism between trees are elaborated. Tree definition is provided, basic concepts of tree traversals are described, details of tree isomorphism algorithms are given, putting special attention and focus on unordered tree isomorphism, algorithm by Valiente as well as algorithm by Aho, Hopcroft and Ullman. Algorithms have been implemented and compared. Comparison was done upon the times needed for each of the algorithms to determine the isomorphism between randomly generated trees. Trees were classified in groups depending on the number of vertices and both algorithms were executed over each group of vertices.