It has been documented since the Renaissance that an air bubble rising in water will deviate from its straight, steady path to perform a periodic zigzag or spiral motion once the bubble is above a critical size. Yet, the unsteady bubble rise has resisted quantitative description, and the physical mechanism remains in dispute. Using an anumerical mapping technique, we for the first time find quantitative agreement with high-precision measurements of the instability. Our linear stability analysis shows that the straight path of an air bubble in water becomes unstable to a periodic perturbation(a Hopf bifurcation) above a critical spherical radius of R=0.926 mm, within 2% of the experimental value. While it was previously believed that the bubble’s wake becomes unstable, we now demonstrate a new mechanism, based on the interplay between flow and bubble deformation.