Some of the most common causes of the condition are allergies to things like pollen, furry or feathery animals or house-dust mites.
Symptoms can include coughing, wheezing, a tight chest and getting short of breath.
About 3.4 million people in the UK have asthma, including 1.5 million children. Find out more with this week's WebGuide. Check Up Practical advice on all aspects of asthma, such as how to find out exactly what you are allergic to, and what to do in the event of an attack. The full transcript of the programme features Dr Mark Britton, a consultant chest physician, answering questions on a wide range of topics connected with asthma. There's also a report on how environmental factors may be increasing the incidence of asthma amongst children.