The paper discusses the background of various seakeeping tools, ranging from strip theory to sophisticated 3d Navier-Stokes solvers, in the perspective to sketch a sort of users guideline for efficient exploitation. In fact, there is no approach that is suited for all seakeeping applications, but their associated advantages and disadvantages should be understood and tools should be applied intelligently, often combining them to have appropriate response time and accuracy in predictions. After a description of approaches (and most prominent codes based on these approaches), recent applications are shown for unconventional craft and offshore/coastal structures involving strong nonlinearities, as demonstration of challenging cases at present.