We will review the main features of the 3-3-1 models. These sort of models are interesting proposals for the physics beyond the standard model because they are free of anomalies only if the number of families or generations is three. Usually, the models are classified according a parameter (denoted by β) appearing in the electric charge operator.
In particular, we show that the β parameter is not enough to differentiate models with different representation content. For instance, in some of them having the same β, but with different particle spectrum, an automatic Z_2 symmetry imply the existence of stable charged or neutral leptons and long lived hadronic resonance. We also comment on the possible existence of a Landau pole in one of the gauge couplings.