A new product, full of possibilities in the field of construction, has now become generally available. This is the group of Epoxi Resins. Most of its applications have proved highly successful, and this makes it essential to study this material in all its aspects. However, in this article only a brief account of the nature of these resins is intented. A history of the discovery is given, and also of its applications to various fields of technology. The article mentions some of the properties of this material, its mode of preparation, and special attention is given to its use in conjunction with mortars, concrete joints, etc. Finally, some information is cited on the tests and research work now being carried out at the Instituto Eduardo Torroja on these resins.