ABSTRACT: Paradigms and theories of how hydrologic modifications caused by dams alter the ecological dynamics of rivers are largely based on studies of temperate basins (e.g. Poff et al. 1997). Little is known about biotic responses to hydrologic modifications in tropical streams: regional generalizations about dam effects in the tropics are constrained by limited data on recently constructed dams and fewer numbers of dams (relative to the large numbers of established dams in temperate zones). General ecological under-standing of effects of dams in both tropical and temperate zones is also constrained by a lack of baseline information on the distribution and ecology of aquatic biota before dam construction, and an overemphasis on economically important species. This article has two main objectives: to examine what is known about regional effects of hydrologic modifications in temperate and tropical areas of the New World (i.e. North and South America and the Caribbean), with an emphasis on fishes and mollusks because of the large amount of evidence that has accumulated on direct effects of hydrologic modifications on these biotic groups at landscape and regional scales; and to discuss research needs regarding regional effects of hydrologic alterations in temperate and tropical regions. A better understanding of regional effects of cumulative hydrologic alterations should be helpful in decisions about the nature and location of future hydrologic modifications. We begin with a brief description of the scope of hydrologic alterations in the New World, emphasizing dams. This is followed by a summary of biotic patterns that have emerged in hydrologically altered rivers draining temperate regions. We use the highly regulated Mobile River Basin in southeastern North America as a temperate-zone case study to discuss specific biological effects. We then focus on the vulnerability of the biota of neotropical rivers and discuss biotic patterns that are emerging in response to relatively recent hydrologic modifications. The Plata River Basin of South America provides a tropical case study. (The term tropical is used to refer to the equatorial area (~30o north and south of the equator) between the northern and southern subtropical dryland zones.) We end by examining research needs and gaps in our understanding of the ecological effects of hydrologic modifications on landscape and regional scales in the New World.