Most modern wireless communication systems employ variable-envelope modulations, causing the Power Amplifiers to work at low efficiency levels. Several solutions to this have been proposed in the literature and the Doherty Power Amplifier (DPA) is one of the most promising ones. Despite being introduced in 1936, the efficiency enhancement scheme devised by Doherty can be applied to modern FET-based RF Power Amplifers with good results. This works explores the problem of the low efficiency levels in backoff and how the active load modulation can be used to counter this effect. Theory about the conventional two-device Doherty is developed and extended to the three-device case, considering two different implementations.