A U‐statistic, calculated from a random sample of size
n
, is an average of a symmetric function of
m
sample values, calculated for all
m
‐tuples in the sample. Examples are the sample variance, the Cramér‐von Mises statistic of goodness of fit, and the Kaplan–Meier and Nelson–Aalen estimators in survival analysis. Asymptotic properties are described.