Aim: This thesis aimed to understand the patterns of skin disease leading to the diagnosis of psoriasis in primary care setting in the UK; develop expert-agreed diagnostic criteria for chronic psoriasis (chronic plaque psoriasis); subsequently applying these criteria to develop a training tool to improve psoriasis diagnosis by non-dermatologists. Methods: Two case-control studies were undertaken involving participants from a large primary care electronic health record database, the Clinical Practice Research Datalink. Individuals with a record of psoriasis within the study window (01/01/2010â29/12/2017) were matched to comparison patients with no previous record of psoriasis based on age, sex, and general practice. Healthcare events including differential diagnoses, clinical features and prescribed medications were examined and their annual incidence rate (IR) and incidence rate ratio (IRR) with 95% confidence interval (95% CI) for ten years before the index date (date of psoriasis diagnosis for cases) were compared between cases and controls. The frequency of GP consultations was also compared between both groups. To improve psoriasis diagnosis, an international panel of 50 dermatology experts took part in three rounds of data collection to establish a clinical diagnostic tool for chronic plaque psoriasis in adults using consensus methods (e-Delphi survey). Subsequently, a training tool based on the findings from the e-Delphi exercise was developed to improve psoriasis diagnosis by non-dermatologists. A before-and-after exploratory investigation of the online training was undertaken with 60 primary care professionals to investigate the impact of training on improving diagnostic skills for psoriasis. Results: 17,320 psoriasis cases and 99,320 controls were included from CPRD GOLD, and 11,442 cases and 65,840 controls were extracted from CPRD Aurum. Data from CPRD GOLD showed that people with psoriasis were up to eight times more likely to be diagnosed with pityriasis rosea at six months (IRR 7.82 (95%CI 4.09-14.95)) before the index date than controls. Cases were twice as likely to be diagnosed with eczema 1.90 (1.76 -2.05), or tinea corporis 1.99 (1.74-2.27) one year before diagnosis. Cases were also more likely to report certain clinical features suggestive of psoriasis (including dry skin, rash, skin texture changes and itching) than controls up to five years before the index date. The most frequently reported clinical feature was rash with IRR of 2.71 (2.53-2.92) at one year before diagnosis. Psoriasis cases were prescribed topical corticosteroids 1.97 (1.88-2.07) or topical antifungals 1.92 (1.78-2.07) in the year before diagnosis twice as often as controls. Data from CPRD Aurum showed similar results to CPRD GOLD. The international e-Delphi exercise yielded two main outcomes: (1) a definition of chronic plaque psoriasis; and (2) nine clinical diagnostic criteria to be used together when making a diagnosis of chronic plaque psoriasis. Diagnostic criteria were further categorised as one essential and eight supportive criteria. Panel ratings indicated that at least four supportive criteria must be present, together with the essential criterion to make a diagnosis of chronic plaque psoriasis in adults. For the training tool study, a convenience sample of 60 primary healthcare professionals (GPs, nurses and pharmacists) completed the training. Findings suggest that the newly developed e-learning tool for psoriasis improved the diagnostic ability of primary care practitioners and that the diagnostic ability of GPs was on average, higher than nurses and pharmacists. After training, participants reported being more confident in making a diagnosis of psoriasis. Conclusions: Potential opportunities for the earlier diagnosis of psoriasis were identified from the medical records of patients with the disease. Earlier diagnosis of psoriasis may be achieved by the following consensus-agreed clinical diagnostic criteria for psoriasis.