For the past 150 years modern artificial cements have slowly been replacing traditional lime based mortars and plasters, to such a degree that now virtually all construction is carried out using only modern materials. Whilst many of these materials are perfectly suitable for modern buildings they have been found to be incompatible with the construction of old buildings. The need to understand the different technology involved in historic and modern structures is essential if successful repair and maintenance programmes are to be carried out. Lime has been the primary binder used in mortars and plasters for a thousands of years and the vast majority of all buildings constructed before 1900 made use of lime. Despite this, in many cases today lime is ignored, so why Lime? Modern cements are harder and less permeable than lime mortars, the general aim when selecting a traditional mortar or plaster is that it should breathe more freely than the material which it is applied to and that it should have less composite strength than the substrate with which it is used. This is essential if you are to prolong the life of the historic fabric.