The current dominant tendency in Europe is to use ancient techniques and materials for repair of ancient buildings. For rendering and repointing this leads to the use of lime mortars, thus respecting historical and architectural authenticity and compatibility with ancient masonry in their chemical, physical and mechanical properties. In the last few years the authors have acquired some experience in the use of lime mortars for ancient masonry walls, making recommendations for application supported by experimental studies. The most important concerns of Portuguese designers are mix preparations, their resistance and durability and the ability to protect walls against rain. In this paper these properties are analysed and the performance of lime mortars is compared with cement and cement:lime mortars. To assess their relative performance, tests to evaluate the ability to protect against water, stresses in the substrate mechanical resistance and durability were carried out on mortar samples of 3 types. Test methods and results are described and discussed.