Optimism and pessimism are complementary, but opposing, dispositions. In general, optimists consistently expect good outcomes and pessimists consistently expect bad outcomes. These variations in human behaviour have profound mental and physical implications. The aim of the present work is to provide an overview of the psychology of pessimism. It is composed of four sections - I: How are optimism and pessimism measured?, II: What is the relationship between optimism and pessimism?, III: What is the relationship between optimism-pessimism and health?, and IV: What are the biology and chemistry of optimism-pessimism? For various reasons, rigorous analysis of pessimism has been difficult. Nonetheless, recent advances, including the development of numerous psychological batteries, have allowed for structured, scientific investigation. Indeed, several studies have examined the medical implications and biological and chemical bases of pessimism. Nonetheless, the psychology of pessimism is imperfect and further study is needed.