Moss occurs in the lawn as a result of constant, excessive moisture and shade. Over time, the moss spreads and the lawn thins. To correct the problem you must change the conditions. Constant excessive moisture may be the result of poor drainage or too much shade. Correcting a drainage problem may be as easy as bringing in some topsoil. Correcting a shade problem may prove to be more difficult if the shade is being caused by a house or large trees. In any case, the first thing to try would be a shade tolerant grass cultivar. This is a type of grass seed specifically designed to grow in shady areas and is available from Brett Young Seeds which is located on the South Perimeter Highway.