  $\alpha$ particle condensation is a novel state in nuclear systems. We
briefly review the present status on the study of $\alpha$ particle
condensation and address the open problems in this research field: $\alpha$
particle condensation in heavier systems other than the Hoyle state, linear
chain and $\alpha$ particle rings, Hoyle-analogue states with extra neutrons,
$\alpha$ particle condensation related to astrophysics, etc.
