The present study deals with flexible structures which protect human lives and buildings against falling rocks with a high energy, up to 5 000 kJ. These rockfall kits have to match the new European standard ETAG 027 which requires different specific full scale tests. To reach high level of performance, these flexible barriers have to be studied in details. First, each main component such as net sets and dissipating devices, have been studied separately. The Anti Sub Marine net was then chosen for its elasticity and isotropic behavior. The best parameters for braking systems have been defined by different tests. Then, the complete design of the structure was performed according to several criteria such as the position of brakes, the post height, the easy repair, the independence of modules and the final cost (kit with its global installation). Finally, the design which fit the best of our criteria was used to perform full scale tests. After some trials, the so-called HC RP® 3 000 kJ and 5 000 kJ kits matched the ETAG 027 requirements. All kits need lateral anchors to stabilize lateral posts during the impact. All the posts are H-shaped and a central net brake is used.