[en] The advantages and the limitations of the transfer reaction approach will be discussed using the results from neutron and proton pick-up and stripping reactions. The strong selectivity in angular momentum transfer, the exploration of a large excitation energy range, the use of polarized beams and the new decay studies help us to establish the main features of the single-particle (hole) strength functions (identification, centroid energy, spreading mechanisms). The problem raised by the subtraction of the underlying background, the method used to extract the strength distributions, the assumptions made to describe the reaction process (DWBA, form factors, unbound nature of the state) as well as the case of overlapping subshells will be examinated