A wheelchair is provided with front anti-tip wheels adapted to assist the wheelchair in mounting an obstacle that the wheelchair approaches obliquely. The anti-tip wheels may function as casters, capable of rotating about a vertical axis, or may be fixed for rotation about a horizontal axis only. The anti-tip wheels may include lateral convex surfaces which tend to allow the anti-tip wheel to slide or roll over the obstacle. The lateral convex surfaces may be provided by a laterally oriented rolling element, a convex-shaped hub, a convex-shaped housing that partially surrounds the wheel, or a ball-shaped wheel. Alternatively, rotation of the anti-tip wheel may be controlled such that the wheel rolling surface faces and engages the obstacle.