This report is the result of the Master thesis of the author, at Delft University of Technology, Faculty of Civil Engineering. The study was performed at DELFT HYDRAULICS, in cooperation with Boskalis Westminster Dredging BV. Despite the many studies and experiments that have already been performed on the subject of low-crested breakwaters, one has still not yet been able to completely comprehend wave transmission and damage inflicted on breakwaters. The knowledge of the processes occurring at lowcrested breakwaters is still limited. Moreover only a small number of experiments is available. This two-part report is an attempt to contribute to the enhancement of the understanding of the wave transmission at low-crested structures and the stability of low-crested breakwaters with an armour layer of Tetrapods. Part A of this report deals with the derivation of new transmission formulae, Part B deals with stability of Tetrapods at the three segments, Front, Crest and Rear of a low-crested breakwater.