  We motivate the usage of finite groups as symmetries of the Lagrangian. After
a presentation of basic group-theoretical concepts, we introduce the notion of
characters and character tables in the context of irreducible representations
and discuss their applications. We exemplify these theoretical concepts with
the groups S_4 and A_4. Finally, we discuss the relation between tensor
products of irreducible representations and Yukawa couplings and describe a
model for tri-bimaximal lepton mixing based on A_4.
