The cohort study on mobile communications (COSMOS) aims to investigate whether there is a link between the use of mobile phones and long-term health problems such as cancer. The international study will run for 20-30 years and will follow the health of at least 250,000 participants aged 18-69 in five European countries. Through four major mobile phone operators, the COSMOS project team from Imperial College London is inviting 2.4 million mobile phone users in the UK to take part in the study. Participants who agree to take part in the study will complete an online questionnaire about their mobile phone use, health and lifestyle. The researchers will monitor participants' mobile phone use and any health problems they might develop, including cancers and neurological diseases such as Alzheimer's disease. They will also analyze whether any changes in the frequency of symptoms, such as headaches and sleep disorders, are related to mobile phone usage.