Complex swarms of mafic dikes extend across New England and Atlantic Canada. Radiometric dates, distributions and structural patterns, petrologic correlations, and geochemical analyses of the postmetamorphic dikes and related plutonic complexes are used to distinguish at least four petrogenetic groups, or igneous provinces, that span the Mesozoic. These groups are as follows. (1) The Early Cretaceous New England—Quebec igneous province, which contains thousands of lamprophyric dikes and at least 20 associated plutonic complexes in northern New England and southern Quebec. (2) The Early to Middle Jurassic White Mountain magma series in central and northern New Hampshire, which contains numerous alkalic diabase and lamprophyre dikes associated with large syenitoid-granitoid plutons. (3) Quartz tholeiitic dolerites of the Early Jurassic Eastern North America dike province, occur as scattered, large dikes that also fed flood basalts, remnants of which are found in the Mesozoic basins. (4) Olivine diabase dikes are present along southeastern coastal New England (CNE groups), for which K-Ar dates and an association with the Agamenticus complex in Maine indicate Triassic ages. The CNE dikes may be correlated with Triassic intrusions in parts of Atlantic Canada (AC groups) as well as with undated dikes in southern New England.