Blood clots happen when substances in the blood clump together as a result of injury to a blood vessel. The substances, called platelets, "plug" the damaged area to prevent blood leakage. Trouble or complications happen when the clot creates a blockage. Clots create other potential problems when bits of them break off and travel to other parts of the body. When clots travel to other parts of the body and cause a blockage, there is the potential for a stroke, clots in lungs (called pulmonary embolism) or clots in the deep veins of the body (called deep vein thrombosis). Follow the links below to find WebMD's comprehensive coverage about what causes blood clots, what they look like, how to treat them, and much more.