A problem of long standing fundamental and practical importance in fluid mechanics is that of the instability and breakup of cylindrical fluid bodies. These problems are encountered in numerous industrial atomization, dispersion, and mixing operations. In some cases, drops are deformed by external flow fields into long cylindrical threads which subsequently break into smaller droplets. In others, liquid jets are injected into air or other liquids and disintegrate into small droplets. Solutions are obtained for critical growth rates and wave-numbers.