The forests of Maryland have been mapped at a scale of 1:250 000 on the basis of 15 regional associations: (1) tamarack (2) bald cypress (3) hemlock—yellow birch—black birch (4) river birch—sycamore (5) sycamore—green ash—box elder—silver maple (6) shingle oak (7) chestnut oak—post oak—blackjack oak (8) chestnut oak—bear oak (9) chestnut oak (10) loblolly pine (11) basket oak—loblolly pine (12) willow oak—loblolly pine (13) basket oak (14) sugar maple—basswood and (15) tulip poplar. The associations differ from each other in species composition and in abundances of species common to many associations. Each was identified in the field by the presence of relatively few common discontinuous tree species referred to as characteristic species. Correlations between forest associations and geologic topographic and soils units mapped at a similar scale suggest that patterns of available water are important in controlling distribution of woody species throughout Maryland.