Chronic lymphocytic leukaemia (CLL) is characterised by the clonal proliferation and accumulation of CD5+CD23+ B cells (lymphocytes) in the bone marrow, blood, secondary lymphoid tissues and spleen (Zhang & Kipps, 2014; Hallek, 2017). This section will cover more recent understanding of the factors involved in the pathogenesis and progression of CLL, especially in relation to the molecular and genetic aspects, signalling mechanisms and role of the microenvironment.