Present state of polaron theory

Some ideas resulting from the work of the present authors and which constitute a part of a paper presented at the "Dag voor de Fysika van de Vaste Stof, Journée Belge de l'Etat Solide, Brussels, December 1964", are exposed here. The importance of separating internal motions from the translational motion of the polaron is emphasized. The relation between polaron momentum and total momentum is considered. Furthermore, the different types of excitations of the polaron are investigated and aIso their possible importance for the mobility problem is mentioned.