Lazy, busy or wildlife-friendly gardeners can let fallen leaves lie where they land in beds and borders, and under hedges. The worms will eventually bring them down into the soil, benefiting its structure. However, lift them away from grey-leaved plants, which may rot from the extra moisture. Clear them off paving for safety's sake and off the lawn for the lawn's sake: big leaves such as sycamore and chestnut are especially smothering to grass. Make sure to take them out of ponds and pools as their decomposition releases gases that are harmful to fish.
If you've a few leaves, mix them into the compost heap. If you've a lot, make leafmould by stuffing them into a cage of chicken wire or into black plastic sacks punctured all over with holes. The leaves will slowly rot down to make a lovely soil and compost additive.