An overview is given on recent mass and g-factor measurements with extreme precision
on single or few cooled ions stored in Penning traps. On the one hand, mass
measurements provide crucial information for atomic, nuclear and neutrino physics as
well as for testing fundamental interactions and their symmetries. On the other hand, g-factor measurements of the bound electron in highly charged hydrogen-like ions allow
for the determination of fundamental constants and for constraining Quantum
Electrodynamics. For example, the most stringent test of CPT symmetry in the baryonic
sector could be performed by mass comparison of the antiproton with H- and the
knowledge of the electron atomic mass could be improved by a factor of 13.