[en] Be¹⁰ has long been of interest for cosmic ray propagation, because its radioactive decay half-life is well matched to the expected cosmic ray age. Recent beryllium isotope measurements from satellites and balloons have covered an energy range from about 30 to 300 MeV/nucleon/sup 1-3/. At the lowest energies, most of the Be¹⁰ is absent, indicating a cosmic ray lifetime of order 2 x 10⁷ years and the rather low average density of 0.2 atoms/cc traversed by the cosmic rays. At higher energies, a greater proportion of Be¹⁰ is observed, indicating a somewhat shorter lifetime. These experiments will be reviewed and then compared with a new experiment covering from 100 to 1000 Mev/nucleon⁴. Although improved experiments will be necessary to realize the full potential of cosmic ray beryllium isotope measurements, these first results are already disclosing interesting and unexpected facts about cosmic ray acceleration and propagation