Although cognitive biases are not particular to the field of medicine, they do in fact influence clinical practice in important ways. Cognitive biases, acquired during life experiences, can unconsciously influence the approach to diagnosis and therefore need to be guarded against. In this chapter, a selection of cognitive biases is presented and organised into categories that are intuitive and easy to remember. As representative examples illustrating these biases, two common conditions—asthma and pulmonary thromboembolism (PTE)—have been dealt with.