This observational study aims to provide new insights into the nature and classification of severe mental illness and dementias that in time should help improve diagnostic practise, and enable the development of new and improved treatments. The investigators will achieve aims by gathering information and biological samples from over 50,000 research participants and then linking this information with participants' electronic health records, genetic and other potential markers of mental illnesses (called biomarkers, derived from biological samples). The investigators will use this resource to analyse how potential risk factors - genetic, other biological and non-biological (related to the participants' life circumstances) - influence participants' experiences, symptoms, and outcomes (both mental and physical health). The investigators will also use advanced analysis to assess whether there may be better ways of grouping together and understanding the experiences of those with severe mental illnesses and dementias. Given the value and importance of this resource for advancing mental health research, the investigators will also make the data available to other researchers to pursue these broad research aims.