We transform the concept of p-summing operators, 1 less than or equal
to p < infinity, to the more general setting of nonlinear Banach space
operators. For 1-summing operators on B(Sigma, X)-spaces having weak
integral representations we generalize the Grothendieck-Pietsch domina
tion principle. This is applied for the characterization of 1-summing
Hammerstein operators on C(S, X)-spaces. For p-summing Hammerstein ope
rators we derive the existence of control measures and p-summing exten
sions to B(Sigma, X)-spaces.