Bows must be periodically rehaired, an operation usually performed by professionals rather than by the instrument owner. Rehairing is done when too many of the hairs are broken, or the hair is dirty, or has lost its friction. Sometimes changing the whole bow can be easier and cheaper than rehairing the old bow, especially with small fractional sized bows.
One way to prolong the useful service life of the hair is to clean it with shampoo or alcohol. Alcohol can damage the finish of the stick if care is not taken to protect it. Contamination of the horsehair with oil, grease, wax, or soap will make it lose its friction, leading to an uneven sound, or no sound at all.
Bows sometimes lose their correct camber (see above), and are recambered using the same heating method as is used in the original manufacture.
Lastly, the grip or winding of the bow must occasionally be replaced to maintain a good grip and protect the wood.
These repairs are best left to professionals, as the head of the bow is extremely fragile, and a poor rehair, or a broken ivory plate on the tip can lead to ruining the bow.

 Bow bugs (anthrenus museorum) are first cousins to the moth and magically appear in unopened cases. Their favorite food is rosin and bow hair (and sometimes gut strings). How can you tell if you have bow bugs? If you open your case and the hair of your bow looks like someone cut it with a pair of scissors, you've probably got bow bugs.
     For infested cases, vacuum the case well, spray with a bug repellent and place the case in sunlight (bow bugs hate light) for a couple of days. We also suggest placing a cedar block inside the case (or mothballs if you don't mind the smell) to kill any remaining bugs.
    If you have to store your bow for an extended period of time, hang it on a wall, away from direct sunlight.