Gutters basically direct rainwater away from your home to the ground below. They prevent water from getting into your basements or crawlspace and protect windows and siding, especially wood, from extreme water penetration. Gutters can be made either from a steel and aluminum combination or from vinyl. While gutters do protect your home, they can sometimes grow mildew which should be removed so that it does not spread to the house, causing damage or even health issues.
Using a power washer, clean your gutters to get the initial mildew and grime off. If you haven't cleaned the debris out of your gutter yet, do this first with a garden trowel.
Use bleach water. Using a soft bristle brush, scrub with one part bleach and four parts water solution. You can also use a bleach spray, like Clorox with bleach.
Use an extended gutter brush to clean downspouts, being sure to disconnect them carefully.
Rinse your gutters with a hose.
Check in a week and periodically thereafter to be sure the mildew has not returned. If it becomes a persistent problem, you may wish to contact a professional.