Recently geotechnical engineers aim to adopt more environmental-friendly solutions (not harmful to the environment), therefore the interest on the use of vegetation as a measure to improve slope stability is increasing. The mechanical reinforcement due to roots against shallow landslides occurs when the fibres intersect the shear surface, usually at depths lower than 2 m. In the literature, the presence of roots is often taken into account by modelling the soil as an equivalent composite material: 'the root-permeated soil', by including an additional cohesion term in the Mohr-Coulomb equation. The models used to estimate the root additional cohesion are presented in the first part of the paper. In some cases, root cohesion is calculated based on the resistant properties of the fibres and assuming an order for the progressive roots failure, either breaking, slipping out or buckling. On the other hand, some authors used structural models of the roots investigating not only the stresses in the roots, but also in the surrounding soil to obtain a better estimation of the root cohesion. In the second part of the paper, the calculation of the root reinforcement is used to assess the safety factor (SF) of the slope. Both Limit Equilibrium analyses (LE) and Finite Element Methods (FEM) are discussed, stressing the limitations of both the approaches.