This paper takes up the systematic study of the Gottlieb groups G n+k (S n ) of spheres for k ≤ 13 by means of the classical homotopy theory methods. We fully determine the groups G n+k (S n ) for k ≤ 13 except for the 2-primary components in the cases: k = 9, n = 53; k = 11, n = 115. In particular, we show [ι n , η 2The Gottlieb groups G k (X ) of a pointed space X have been defined by Gottlieb in [9] and [10]; first G 1 (X ) and then G k (X ) for all k ≥ 1. The higher Gottlieb groups G k (X ) are related in [10] to the existence of sectioning fibrations with fiber X . For instance, if G k (X ) is trivial then there is a crosssection for every fibration over the (k + 1)-sphere S k+1 , with fiber X . This paper grew out of our attempt to develop techniques in calculating G n+k (S n ) for k ≤ 13 and any n ≥ 1. The composition methods developed by Toda [36] are the main tools used in the paper. Our calculations also deeply depend on the results of [13,16,21].