Book excerpt: Depending on the health profile of the traveller, the type of travel to be undertaken, and the place of transit and destination, travellers may face various health risks during travel. The International travel and health collection serves as an entry point for other World Health Organization (WHO) publications that provide further information. Its primary target audience is travel health practitioners and travel health professionals, who provide health advice to travellers on appropriate precautions to be taken to minimize any travel-related health risks in unfamiliar environments, before, during and after travel. Module 5 lays out the various aspects relevant to mental, neurological and substance use (MNS) conditions that travel health practitioners should be aware of when supporting travellers, or people with these conditions who are considering travelling. International travel can be a stressful experience. Travellers may face separation from family and familiar social support systems as well as having to adjust to foreign cultures and languages. Coping with high levels of stress may result in physical, social and psychological problems. Changes to the circadian rhythm and sleep deprivation can trigger seizures in people with epilepsy, provoke migraine attacks and exacerbate behavioural symptoms in people living with dementia. Under the stress of travel, pre-existing MNS conditions may be exacerbated or become apparent for the first time.