Biologic therapies have been licenced for use in children and young people with juvenile idiopathic arthritis (JIA) from the early 2000s, with an increasing number available from 2011 onwards. At the outset of this work, how these drugs were being used in JIA in routine practice beyond clinical trials was unknown. There was also little information available regarding their effectiveness in routine use. Knowledge of adults with JIA starting biologic therapy after childhood was even sparser. The aim of this PhD thesis was to describe prescribing patterns and effectiveness of biologic therapy in children and adults with JIA using data from real-world cohorts in the United Kingdom (UK). Six publications are presented. The prescribing patterns of biologics in children and young people with JIA were investigated in publication 1. The effectiveness of biologics in children with JIA, including the effect on disease activity and growth, was investigated in publications 2-5. The effectiveness of biologics in adults with JIA was investigated in publications 6. The publications use data from three UK observational studies: the British Society for Paediatric and Adolescent Rheumatology Etanercept Cohort Study (BSPAR-ETN) and the Biologics for Children with Rheumatic Diseases (BCRD) study, both focusing on children with JIA, and the British Society for Rheumatology Biologics Register for Rheumatoid Arthritis (BSRBR-RA) which includes adults with rheumatic conditions. The work presented in this thesis demonstrates that in clinical practice, there is (i) stratification of children and young people with JIA on to different biologic therapies based on their disease characteristics, (ii) the majority of children and young people are achieving good outcomes on etanercept therapy, both regarding disease control and vertical growth, (iii) there is no difference between effectiveness outcomes in children and young people with systemic JIA treated with tocilizumab versus anakinra, (iv) rituximab, whilst untested and unlicensed, is used in children and young people with JIA and appears to be an effective treatment, and (v) tumour necrosis factor inhibitor therapy is an effective and safe treatment choice in adults with JIA despite a delay following disease onset, with similar outcomes in disease activity, functional ability, and safety profile compared with those seen in rheumatoid arthritis. This thesis presents these six publications as a body of work including their place in the medical literature as well as a critical discussion of the methodological approaches used.