Dehydration occurs when you use or lose more fluid than you take in and your body doesn't have enough water and other fluids to carry out its normal functions. If you don't replace lost fluids, you will get dehydrated. Not drinking enough water in hot weather may cause dehydration. Anyone may become dehydrated, but young children, older adults and people with chronic illnesses are most at risk. Learn the symptoms and when you should see a doctor.