The whole story: Winter rains and snow turn fluffy layers of leaves into dense, soggy mats that can kill your grass by denying it oxygen and encouraging disease. Even during dry winters, a thick layer of leaves on your lawn blocks sunlight and reduces air circulation. If you shred your leaves into small pieces with your mower, you can leave them where they fall without suffocating your lawn.
A mulching mower—one fitted with a blade that chops leaves and grass clippings into small pieces—does the job best, but a side-discharge mower works, too. Get ready to shred by setting the mower height to 3 inches and removing the bag. It's best to shred leaves when you can still see some grass peeking through them, which means that you may need to pull out the mower more than once this fall if you have big trees.
Begin mowing on the outside edge of your lawn, making sure that you shoot the leaves toward the middle of the yard. Mowing in this pattern also allows you to mow over the leaves more than once and keeps them from ending up on your sidewalks. If the leaves are still in fairly large pieces after your first pass, go back over the lawn at a right angle to your first cut. Finely shredded leaves filter down through the grass and easily decompose by next spring.
If a thick layer of shredded leaves buries your lawn, you must suck up the extra leaves by making one more pass over the lawn with the mower's bag attached. You can also mow with the bag on if you want to collect leaves for your compost pile, or to use as mulch in your garden beds. It's best to have no more than a 1-inch layer of leaf mulch on lawns and a 3-to-4-inch layer on garden beds. Mulched leaves return valuable micronutrients to your lawn and gardens (especially when mixed with grass clippings) and feed the microorganisms and worms that keep your soil—and your grass—healthy.