Abstract: This seminar talk reports on high-precision mass measurements of 99-101In isotopes and isomers with the ISOLTRAP mass spectrometer at ISOLDE/CERN. Applying the Multi-Reflection Time-of-Flight (MRToF) technique, the masses of 99In and 100In (the β-decay daughter of 100Sn) were measured for the first time with high precision. Additionally, the recently implemented Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique will be discussed in the context of these measurements. The former mass-spectrometry technique allowed, for the first time, for a direct mass determination of both the ground and the isomeric states of 101In in a Penning trap with resolving powers exceeding m/Δm > 5·105 in only 62-ms phase-accumulation time. Our mass spectrometry results will be compared with ab-initio many-body calculations in this mass region. The 100-fold improvement in the precision of the 100In mass value highlights a discrepancy in the previously published atomic mass values of 100Sn, which could so far only be derived from β-decay results.