Both CeCu 2Si 2 and YbRh 2Si 2 crystallize in the tetragonal ThCr 2Si 2 crystal structure. Recent neutron-scattering results on normal-state CeCu 2Si 2 reveal a slowing down of the quasielastic response which complies with the scaling expected for a quantum critical point (QCP) of itinerant, i.e., three-dimensional spin-density-wave (SDW), type. This interpretation is in full agreement with the non-Fermi-liquid behavior observed in transport and thermodynamic measurements. The momentum dependence of the magnetic excitation spectrum reveals two branches of an overdamped dispersive mode whose coupling to the heavy charge carriers is strongly retarded. These overdamped spin fluctuations are considered to be the driving force for superconductivity in CeCu 2Si 2 (T c=600mK). The weak antiferromagnet YbRh 2Si 2 (T N=70mK) exhibits a magnetic-field-induced QCP at B N=0.06T (Bc). There is no indication of superconductivity down to T=10mK. The magnetic QCP appears to concur with a breakdown of the Kondo effect. Doping-induced variations of the average unit-cell volume result in a detachment of the magnetic and electronic instabilities. A comparison of the properties of these isostructural compounds suggests that 3D SDW QCPs are favorable for unconventional superconductivity. The question whether a Kondo-breakdown QCP may also give rise to superconductivity, however, remains to be clarified.