Chronic lymphocytic leukaemia (CLL) is characterised by marked disease heterogeneity and is currently incurable. This thesis presents work undertaken to discover novel biological and therapeutic insights through the investigation of spontaneous CLL regression, the evaluation of ATR inhibition as a therapeutic strategy, and the assessment of the impact of post-treatment minimal residual disease (MRD) in CLL. Firstly, spontaneous regressed CLL tumours were found to express somatically mutated IGHV genes, display unresponsiveness to IgM and IgD BCR stimulation and exhibit a phenotype of short telomeres with low CD49d expression, suggesting a model in which the CLL clone undergoes an initial period of proliferation which subsequently subside into a state of anergy and low proliferation. Secondly, ATR inhibition was found to be a promising therapeutic target for CLL tumours with TP53 or ATM defects. Treatment with ATR inhibitor induced synthetic lethality and selective cytotoxicity to these tumours in vitro and in vivo, and sensitised them to chemotherapy and ibrutinib. Finally, MRD negativity was found to predict for 10-year survival in CLL independent of the type and line of treatment, as well as known prognostic factors including adverse cytogenetics, supporting its use as a prognostic marker and potential therapeutic goal in CLL.