Why is it necessary to get your chimney swept?

Many factors affect chimney health including (but not limited to) the age of your home, your chimney type, and the weather (strangely enough, mild weather seems to more drastically affect chimneys than very cold weather.) Another common factor that affects chimneys is the building of nests by birds or small animals within fireplace / chimney flues during Summer months, or dry leaf accumulation during the Autumn.