  Possible generalizations of quantum theory permitting to describe in a unique
way the development of the quantum system and the measurement process are
discussed. The approach to the problem based on the Lindblad's equation for the
statistical operator is reviewed. The Tomonaga-Schwinger like equation of this
type is introduced to establish Lorentz invariance. The application of
tachyonic field to overcome divergences arising in this equation is analyzed.
Other approaches to the problem are shortly discussed.
