  Notes used for a course held in 2016 in the School of Advances in Group
Theory and Applications, for some lectures given in 2018 for the students of
the Master in Mathematics of the Vrije Universiteit Brussels, a course for
master and Ph.D. students at the Universidade de S\~ao Paulo and at the
conference Group algebras, representations and computations. We revise some
problems on the study of finite subgroups of the group of units of integral
group rings of finite groups and some techniques to attack them.
