Oral candidiasis is a common opportunistic oral infection in humans caused by overgrowth of Candida species, in particular Candida albicans. Clinically it usually presents as pseudomembranous or erythematous candidiasis. It may be asymptomatic or associated with local discomfort, dysgeusia and xerostomia. The most common risk factors for oral candidiasis include treatment with antibiotics, poor oral hygiene, tobacco smoking, denture wearing and salivary gland hypofunction. A large number of diseases as well as their treatment including diabetes, cancer and cancer therapy, HIV infection and treatment with immunosuppressants are associated with oral candidiasis. In immunocompromised patients, the localized oral infection can spread through the bloodstream or upper gastrointestinal tract leading to severe infection with increased morbidity and mortality. This chapter focuses on Candida as commensal oral microorganism, the clinicopathological aspects in medically compromised patients and diagnostic methods available regarding oral candidiasis.