The phrase ‘complex needs patient’ is often used by clinicians to describe a patient who presents with challenges and needs that require management approaches that are resource intensive and multi-focused. These individuals are often passed from service to service, with high costs to services across the board. In this chapter, we seek to define ‘complex needs patients’, recognising that for many clinicians the phrase refers to those individuals who present with severe mental illnesses together with other comorbid challenges including, but not limited to, serious physical illness, substance misuse or addiction, social problems including a lack of support, homelessness as well as problematic, absent or abusive relationships and the presence of another comorbid mental illness. This chapter explores the possible aetiological factors of complexity as well as its background and characteristics and discusses useful treatment modalities. Lastly, it considers the impact that the Covid-19 pandemic has had both in terms of disease presentation and the impact it has had on services.