Bare spots in your lawn may be caused by heavy foot traffic (compaction), drought, disease, chemical burn and weed or insect infestation.
Your grass holds soil in place to prevent more damage. When bare spots occur where grass is not growing, your lawn becomes susceptible to ruts which may increase in size. If grass is not growing in a particular area, weeds probably will, as they easily adapt conditions causing the bare spots.
The important thing in repairing a lawn is to match the new, with what you already have in place so it's a nice blend. For example, you live in the north and you have a blend of Kentucky bluegrass (very common) and you've got a few dead spots that didn't green-up this past spring, you'll want to plant more Kentucky bluegrass blend. Don't put in tall turf type fescue because you heard it's really great (which it is). They are two different types of grass that shouldn't be mixed.
Now, if the bad areas are small, areas less than a foot in diameter, ignore them, they'll fill by themselves in a month or so. Larger than a foot and then it's time to take action.
Step One: remove the old dead areas with a shovel. Trim up the sides of the area being removed so they're straight. Fill in the area with new top soil, so the new is level with the old. Now plant your seed (follow label directions). In the south, put in matching sprigs or plugs. For seeds, you can cover with some straw, which is good for areas that might be damaged by a heavy rain. Otherwise, just a little peat moss on top will work just fine.
When planting the seeds, don't bury them in the ground. Use just enough top soil to barely cover the seeds (no more than ¼", with less being preferable). After covering, press down on the dry soil with the back of a hoe, or use your shoe and lightly tamp it down, but don't stomp on it.
Options: There are some products out there that combine a few steps and make it a little easier. Scotts PatchMaster contains just about everything you need: seed at a predetermined rate, fertilizer, and mulch to help keep the seeds properly moist. This works great if you happen to have the same grass type as the package. If not, then don't use it.
Another option is if you can find sod that matches your lawn, you can quickly repair the damaged areas. Follow all of the directions up to the point of planting seed, except for the added top soil. Only add enough topsoil so that with the sod, it matches your existing level. Sod takes about 2 weeks to get established. Keep it watered and don't let it dry out during those first 2 weeks.
Step Two: Apply a light application of fertilizer over the area (follow label directions) that is specifically designated as a "starter fertilizer."
Step Three: Keep the soil evenly moist using a fine mist setting. Avoid strong directional spraying as this will dislodge the seeds and cause an uneven growth pattern. For seeds, only the top surface needs to stay moist, but (and this is important, especially if the weather turns hot) it can't be allowed to dry out completely, particularly in the last half of the first 2 weeks after planting.
Once seeds germinate, continue to keep the soil evenly moist and increase the amount of water, but cut back on the number of times you water. In other words, keep the soil moist at a deeper level (moist-not wet!).
Step Four: In a few weeks things will start to pop. If possible, don't walk on the areas, and don't mow the areas until the seeds are about 2-3" tall, about 3 weeks after the seeds first germinate. Then you can forget about it and just treat it like the rest of the area. Don't use any weed controls on the new grass for a couple of months until it gets really established and hardened off.
SPRING: Late spring is the best time to plant and repair worn spots in warm season grasses.
SUMMER: Newly planted patches of grass will need help surviving both heat and drought. Water the new patches of grass regularly and cover them with straw or other mulch in drought conditions.
FALL: Early fall is the best time to repair bare spots in cool season grasses. If you are unable to repair worn patches later in the fall, use erosion mats to stop soil from washing away and to prevent the problem from getting worse.