One of basic characteristics of regular bipedal walk of humanoid robots is the maintenance of their dynamic balance during the walk, whereby a decisive role is played by the unpowered degrees of freedom arising at the foot-ground contact. Hence, the role of Zero-Moment Point (ZMP) as an indicator of dynamic balance is indispensable. The paper gives a very detailed discussion of some basic theoretical assumptions related to the notion of dynamic balance (term stability, is often used as a synonym, which we consider erroneous). Some special cases of gait in which dynamic balance need not be realized are also discussed.