The ratio of protons to neutrons in stable nuclei ranges from 1:1 (light nuclei) to 1:2 (heavy nuclei). By adding more neutrons or protons to a stable nucleus the binding energy of the system decreases until it becomes unstable against proton or neutron emission. These limits are called drip-lines. Mass measurements on very proton- or neutron-rich or isotopes allow to predict where these drip-lines are located.