Idelalisib (Zydelig(®)) is a highly specific small-molecule phosphatidylinositol-3-kinase (PI3Kδ) inhibitor that has been developed as an oral treatment for B cell haematological cancers.It has received its first approval in the US in July 2014 for the treatment of relapsed chronic lymphocytic leukaemia(CLL), relapsed follicular B-cell non-Hodgkin lymphoma(NHL) and relapsed small lymphocytic lymphoma (SLL) [corrected]. Idelalisib is under regulatory review in the EU-where it has received a positive opinion from the European Medicines Agency Committee for Medicinal Products for Human Use-and in clinical development for CLL in Australia and Canada. This article summarizes the milestones in the development of Idelalisib leading to this first approval for relapsed CLL, NHL and SLL.