The field I go to typically has calm winds and clear skies till about noon. If the breeze picks up before then, then its usually less than 5 MPH. Although, one time, I was flying the Ultra Stick .60 in a 25 MPH 90 degree crosswind, and upon landing, I had 50% flaperons, and full right rudder, and the airplane still landed 45 degrees to the center line. Thankfully, since it was into the wind and with the flaperons, it stopped before it hit the safety fence. So, as others have said, it really depends on your experience, the aircraft, and common sense. For example, if you have a foamie, no use in flying in a 20MPH wind...although, during the last fun fly, we had a 20MPH headwind straight down the runway. One of the events was a high low, AKA fly as fast as you can between two marked points on the runway and fly as slow as you can down the runway. Everyone made it, but this one guy brought a foamie to the fun fly. He still had to fly upwind like everyone else, and at full power (this is with a brushless motor and 3 cell lithium polymer), the aircraft would go like one foot forward and three feet back. So, we voted to make an exception to the rules just for him, and he was able to do the high-low downwind. Okay, I'll shutup now.