Increased competition for access to agricultural land in Africa has led to a renewed interest in land markets. While there is a growing literature on the economic effects of markets, in terms of efficiency and equity, little is known about the institutions underlying their operations. Based on a study conducted in the Malagasy Highlands, this thesis addresses the role of local institutions in the functioning of land markets, with a focus on the role of the family which is traditionally involved in “ancestral” land governance. Using mostly first-hand, qualitative and quantitative data, this work is structured around three main topics: the role of local rules in the concrete unfolding of transactions; the allocative effects of transactions in terms of equity; the conflicts that may arise and the institutional devices to secure transactions. The thesis shows that markets expansion does not deprive the family of its land’s prerogatives and that the family actually contributes to organizing land transactions. First, the family is involved in the definition and enforcement of the "right to sell": sales of inherited land require the approval of the family which also implements intra-family priority rules in the sales process. Second, although purchases are accessible only to the wealthiest households who can afford to pay for the price of land, land transactions do not appear to strengthen land inequalities. Indeed, our results suggest that land markets contribute to equalize the distribution of land in favor of households with less inheritance. The mitigation of inequalities stemming from inheritance is partly explained by specific family rules that channel land sales offers to relatively less endowed households. Finally, the thesis explores the interactions between family and land markets through an analysis of conflicts and transaction securing practices. Results show that market transactions can trigger family-induced conflicts. One common conflict type is related to the contestation, by third party belonging to the family inheritance group, of the "right to sell". A second type has to do with the ex post re-interpretation of the terms of a past transaction (the seller claiming that the “sale” was in fact a long-term tenure arrangement). On the other hand, and despite the existence of land transaction registration devices, either formal (titles, certificates) or semi-formal (“piece of papers”), the family continues to play a key role in legitimating sales and securing transactions.