In this final paper, the subject of weldability of high-alloy steel S690QL is dealt with in two parts, theoretical and experimental. In the theoretical part, high-strength steels are described, i.e. their basic division, areas of application and advantages and disadvantages. After getting acquainted with high-strength steels, exactly one steel from that area is described in detail. The fields of application of S690QL steel, its mechanical and chemical properties and the microstructure itself are listed. Some of the most common welding procedures of this steel are described, as well as the dependence on temperature and welding parameters on the appearance of cracks and the microstructure of the welded joint itself. Also, the concept of weldability of steel is explained, as well as the weldability of S690QL steel is described. In the experimental part, two types of welding were performed. In the first variant, two plates of the same basic material, high-strength steel S690QL, were welded in 3 different positions. In the second variant, two plates of different base materials are welded, one of high-strength steel S690QL and the other of non-alloyed steel S355J2C in one welding. After welding, all four samples were subjected to three non-destructive testing methods, visual testing, magnetic particle testing and ultrasonic testing. Testing of the mechanical properties of the weld as well as its microstructure was also carried out. The purpose of these tests was to detect the appearance of possible cracks, and consequently to define the weldability of high-alloy steel S690QL, both alone and with different steels at different parameters and welding positions. The results and conclusions obtained after the conducted tests in combination with the theoretical basis are presented in the following text.