It has long been predicted that the scattering of ultracold atoms can be altered significantly through a so-called ＇Feshbach resonance＇. Two such resonances have now been observed in optically trapped Bose-Einstein condensates of sodium atoms by varying an external magnetic field. They gave rise to enhanced inelastic : processes and a dispersive variation of the scattering length by a factor of over ten. These resonances open new possibilities for the study and manipulation of Bose-Einstein condensates.