The regimes of jet breakup including the first-wind induced, second wind-induced, and atomization regimes are elucidated by use of a single equation. The equation is obtained by the use of linear stability analysis. The equation relates the dimensionless intact length L to the dimensionless velocity U for relevant flow parameters. The results are plotted in the L vs U space to elucidate the regimes and mechanism of the jet breakup. The theory compares well with some experiments.