This book contains introductory comments concerning the definitions, the symmetry properties and the basic formulae for matrix elements of tensor operators. The main body of text consists of tables of reduced matrix elements of double tensor operators within pn, dn and fn configurations, i.e. all the principal pure configurations of the atomic and nuclear structure. These tables are complete which means that the author has run through all the states of the following configurations: p2, p3, d2, d3, d4, d5, f2, f3, f4, f5, f6, f7 for multiparticle systems with central symmetry.Double tensor operators wk_1k2 have been chosen in these calculations, for a number of reasons. First of all, they are the most general type of spherical tensor operators and, since they are normalized, they are the most convenient to use. Secondly, they have been very frequently used in many areas of physics, most notably in nuclear, atomic, molecular and solid state physics. There are also tabulated conversion factors to enable the use of these tables for other types of definitions of tensor operators.