The theory of U-statistics goes back to the fundamental work of
Hoeffding [1], in which he proved the central limit theorem. During
last forty years the interest to this class of random variables has been
permanently increasing, and thus, the new intensively developing branch
of probability theory has been formed. The U-statistics are one of the
universal objects of the modem probability theory of summation. On the
one hand, they are more complicated "algebraically" than sums of
independent random variables and vectors, and on the other hand, they
contain essential elements of dependence which display themselves in the
martingale properties. In addition, the U -statistics as an object of
mathematical statistics occupy one of the central places in statistical
problems. The development of the theory of U-statistics is stipulated by
the influence of the classical theory of summation of independent random
variables: The law of large num- bers, central limit theorem, invariance
principle, and the law of the iterated logarithm we re proved, the
estimates of convergence rate were obtained, etc.