This Ph.D. thesis is presented as three separate papers, and the overall aim of this research is to describe and achieve a broader and clearer understanding of the epidemiology, aetiology and symptomatology of schizophrenia in Saudi Arabia. The first study provides knowledge about the epidemiology of schizophrenia by investigating the incidence in Riyadh, Saudi Arabia and using the incidence data to describe heterogeneity across districts in Riyadh. In addition, the study tests whether variation in incidence occurs according to nationality, sex, age, marital status, employment status, and income. The second study evaluates the five-factor model in Saudi schizophrenia patients by factor analysis of OPCRIT items as rated from the health records. It also tested whether there was any association between the five factors and demographic data included in OPCRIT. The third study describes the duration of untreated psychosis in Riyadh to identify any association between both patient demographic factors and their first pathway to care with their duration pf untreated psychosis (DUP). The chosen study design for the whole research was a retrospective case note study of all incident cases of schizophrenia over a 2 years period presenting in the capital city of Saudi Arabia. The first study is an epidemiological study with an ecological design, which determines the incidence of schizophrenia amongst the population in Riyadh and to identify associations between incidence of schizophrenia and demographic and socio-environmental characteristics. The second study is a Factor analysis of OPCCI items from a total of 421 schizophrenia patients in Riyadh who presented between 2009 and 2011, while the third study a descriptive DUP (duration of untreated psychosis) study which focused on describing the duration of untreated psychosis in and to identify any association between the DUP and both patients demographic factors and their first pathway to care. The results showed that the incidence rate of schizophrenia in Saudi Arabia is similar to those recorded in Western countries with an associations between schizophrenia incidence and younger age, male gender, single status and unemployment. Lack of association between population density and area level income with schizophrenia incidence was also confirmed. The second study produced five-symptom dimensions, mania, depressions, reality distortion, disorganisation, and manic/bizzare delusions explaining 33% of the total variance. Different dimensions were differently associated with the demographic/premorbid risk factors. Results of the third study showed that the median DUP was 1.41 years. Older age at onset, single mariatl status and higher educational were associated with shorter DUP. Long DUP was associated with help seeking from traditional healers. This thesis has presented a comprehensive picture of the epidemiology of schizophrenia in the capital city of Saudi Arabia, duration of untreated psychosis and a factor analysis of symptoms of schizophrenia.