Early detection of cancer is crucial to improve patients’ chances of survival. Unfortunately, early signs of cancer are often easier to see with the benefit of hindsight than they are at the time of presentation. Although cancer is common, cancer diagnosis is a relatively rare occurrence for an individual GP and referral from primary to secondary care is often triggered by a GP's awareness of ‘alarm symptoms’ or ‘red flags’ that are considered to predict malignant disease. It is important to have a lower threshold for investigating cancer in people with recognized risk factors for the disease, such as smoking. Failure to respond to treatment as expected (for example, a chest infection that does not settle with antibiotics), or symptoms that are getting progressively worse should raise suspicion of a possible diagnosis of cancer in any patient.