The present development of conservation works within the programme o f the renewal of historic urban complexes and o f individual structures provides a conducive base for considerations on the complexity o f technical problems encountered in their preservation. A good or bad condition of monuments depends on an adequate or inadequate solution of such problems. A technically satisfactory and faultless solution, which would provide for the protection of the structure as a building, may at the same time not protect it as a historic monument. In this context, the protection of the monument means the preservation of the values that played a decisive role in recognizing it as an architectural monument. Studies on the construction and material problems have been considered as three stages o f the work of the engineer-constructor: 1st stage — studies on a technical condition o f the structure as well as other studies which have to be made under constructor’s surveillance, 2nd stage — planning out conservation works, out o f which the construction part may occupy a leading position, 3rd stage — execution of the project, which calls for a particular accuracy when carrying out the works and for the observance of the principles of building art and of preservation spirit. In the first stage of research works one should not employ methods that might destroy, damage or affect the historic substance. A number o f the examples may be found to show that ineffective methods used hitherto may be replaced with non-destructive ones. Basing on the studies made a factual technical condition is established according to the norms that are in force in designing, which is not always justified, as not all factors are taken into consideration. When planning conservation works, the following construction and building solutions should be taken into account: — use of new constructions presented in a clear and readable way, — maximum preservation of the historic substance and even its inviolability, — to avoid experimental untested solutions that could do harm in future to *he existing substance, , — use of the materials of the best quality and durability. The discussion of the above principles has been supported with examples presented in form o f illustrations and brief descriptions. One o f the major tasks in the execution stage is the sequence of measures and works that would guarantee security in each phase of the works as well as the protection o f a historic building against destructive effects of water. Conclusions: 1. The constructor participating in works on the protection of architectural monuments is and must be a member o f the conservat'on team opeiating in conformity with preservation principles and, first and foremost, with the principle of the preservation of the authentic substance o f monuments. 2. Two notions o f security are associated with historic monuments, i.e. (i) security of the structure as a building meant for a permanent or temporary inhabitation (by people or animals) or as a store of materials and things on the one hand and security o f the structure as a monument exposed to the risk o f losing its authentic monumental substance and historic and artistic values on the other. The duty of the conservator and especially o f the conservator-constructor, is to pay respect to both notions of security. 3. Norms on the construction designing and also norms related to traditional materials should include a law allowing to make statistical calculations based on the assumptions other than those given in norms provided that the attention is paid to the values of the structure as a monument of national culture.