This text represents over 20 years of research on distortions of functionals under actions of linear integral operators. It is divided into two parts. The first part addresses linear integral operators, establishing their properties and attempting to arrive at both specializations as well as generalizations to be used in the second part. The second part is devoted mainly to the development of several kinds of distortions under actions of integral operators for various familiar functionals. Among the topics that are treated are absolute modulus, real part, range, length and area, angular and derivative. Also, distortions on the class of univalent functions and its subclasses, Caratheodory class, and distortions by a differential operator are dealt with.