Recent observation of a "kink" in single-particle dispersion in photoemission experiments on cuprate superconductors has re-initiated a heated debate over the issue of a boson that "mediates" the pairing in cuprates. If the "kink" is indeed caused by interaction with a bosonic excitation, then there are two possible candidates: phonons and spin fluctuations. Here, the role of anti-ferromagnetic spin fluctuations in shaping the phase diagram of cuprate superconductors will be discussed. By using the local (momentum-integrated) dynamic spin susceptibility, recently measured in neutron scattering experiments to high energies, the electronic self-energies are calculated that agree in many aspects with those measured directly in angular resolved photoemission and optical spectroscopies. The spin fluctuations therefore seem to play a role typically played by phonons in renormalizing single particles. The key question emerging from this picture is whether the coupling detected in angle-resolved photoemission spectroscopy (ARPES) reflects the mediating boson, i.e. whether the spin fluctuations may be responsible for superconducting pairing.