  It is evident that models of the knee should match the observational
phenomenology. In this talk I discuss a few aspects of phenomenology, which are
important not only for the understanding of the knee origin, but also for the
general problem of the origin of cosmic rays. Among them are the shape of the
energy spectrum, its irregularity, the sharpness of the knee and its fine
structure. The classification of models is given and some examples of the most
recent models are discussed. The most probable conclusion deduced from this
examination is that the knee has an astrophysical origin and the so called
'source' models of the knee are most likely among them.
