Gutter overflow can be caused by two things. When gutters overflow it can cause serious problems for the foundation and walls of your home. There are two things that make gutters overflow, clogs and sagging.
Clogs in the downspouts or clogs in the gutters themselves are the easiest thing to fix when your gutters overflow / simply get up there, find the clog and remove it. For clogs in the downspout you can usually just feed a hos through the downspout and blast it out with water pressure.
Sagging gutters is a different story. Sagging gutters often make themselves worse because they fill with water and then the weight of the water causes them sag even more. Sagging gutters are sometimes caused by improper installation but mostly they happen when something has changed since the installation. Perhaps the ground underneath the house has settles and causes a tilt to the roofline? Maybe a branch fell on the gutter and bent it? Perhaps a workman placed a ladder up against it and bent it etc etc etc. The fix is to give us a call and have us make all the gutters flow again!