In the international trade literature there seems to be some confusion between "openness" and "protection" measures. The aim of this paper is to bring together the "state of the art" in quantifying trade policy measures, so we focus on the extent of the protection granted by policies rather than on the degree of openness of the economy. Given the huge size of literature dealing with these issues, we limit our review as follows. On the one hand, we focus on trade policies implemented at the border: accordingly, we do not consider all the other possible public interventions influencing trade flows. On the other hand, we take into account only indexes explicitly adopting a metric expressed in a "scalar aggregate" (tariff- and quota-equivalent measures, or an index in a closed interval). We distinguish between indexes that aggregate across products (same barrier for more products) and indexes that aggregate across instruments (more barriers for the same product). Finally, in order to classify the large number of indexes covered in our review, we propose a typology based on three categories: incidence, outcome and equivalence.