Diapause is an event of ecological significance in the lives of a wide range of invertebrates. The underlying physiological and genetic mechanisms pose many questions and have important evolutionary implications. Some of the most dramatic examples of diapause are provided by crustaceans. Most branchiopods produce resting eggs that can withstand intense dessication and extremes of temperature and pressure far beyond those to which they are ever subjected in nature. Furthermore they can remain in a dry condition, displaying no measurable metabolism, for many years. Such abilities enable them to inhabit infrequently-filled pools in arid regions. When the pools fill with water, these dormant eggs hatch and give rise to animals that grow quickly to maturity and produce a new generation of resting eggs. There is every reason to believe that primitive branchiopods have employed diapause in this way since Palaeozoic times and that their abilities in this direction have played a key role in their continued success as freshwater animals.