Mass measurements of fission products based on Penning-trap technique are reviewed in this article. More than 300 fission products have been measured with JYFLTRAP, ISOLTRAP, CPT, LEBIT and TITAN Penning traps with a typical precision of δm/m ≈ 10−7 − 10−8. In general, the results agree well with each other. The new data provide a valuable source of information and a challenge for the future development of theoretical mass models as well as for obtaining a deeper insight into microscopic properties of atomic nuclei as measured, for example, via key mass differentials. Shape transitions around N = 60, subshell closure at N = 40 and shell closures at N = 50 and N = 82 have been investigated in the trend of the precisely measured two-neutron separation energies. The evolution of two-neutron and two-proton shell gaps has been studied and compared to theoretical models for Z = 50, N = 50 and N = 82. Proton–neutron pairing effect in separation energies and odd–even staggering of masses are shortly discussed. In addition to nuclear structure, many experiments have been motivated by nuclear astrophysics.