Concentration is the final stage of the noble eightfold path but is followed by insight and liberation in the tenfold series. It is usually described as achieved through practice of a series of four ever deeper states of mental one-pointedness known as jhānas. This series is often extended to five in later texts and may be followed by a second series of so-called “formless states” culminating in complete “cessation”. This chapter identifies key discrepancies between the fourfold and fivefold series, focussing particularly on the description of first jhāna and how this should be understood. The discussion critically examines the conclusions of other Buddhist scholars as to how differing accounts are likely to have arisen. It concludes that concentration as understood in early Buddhism originally consisted only of the set of four jhānas and that the sequence of “formless states” is therefore almost certainly a late doctrinal addition.