I review the basic problems posed by the existence of the Fundamental Plane, and discuss its relations with the Virial Theorem (VT). Various possibilities are presented that can produce the observed uniform departure from homology (structural, dynamical) and/or from a constant stellar mass-to-light ratio. The role of orbital anisotropy and its relation with the FP thickness are also discussed. None of the explored solutions — albeit formally correct — are easily acceptable from a physical point of view, due to the ever-present problem of the required fine-tuning.