In the conifer genus Juniperus (Cupressaceae), many species are increasing rapidly in distribution, abundance, and dominance in arid and semi-arid regions. To help understand why junipers are so successful in drier habitats, we studied hydraulic traits associated with their water stress resistance, including vulnerability to xylem cavitation, specific conductivity (KS), tracheid diameter, conduit reinforcement, and wood density in stems and roots, and specific leaf area (SLA) of 14 species from the United States and the Caribbean. A new phylogeny based on DNA sequences tested the relationships between vulnerability to cavitation and other traits using both traditional cross-species correlations and independent contrast correlations. All species were moderately to highly resistant to water stress-induced cavitation in both roots and shoots. We found strong phylogenetic support for two clades previously based on leaf margin serration (serrate and smooth). Species in the serrate clade were 34% to 39% more resistant to xylem cavitation in stems and roots than were species in the smooth clade and had ~35% lower KS and 39% lower SLA. Root and stem resistance to cavitation and SLA were all highly conserved traits. A high degree of conservation within clades suggests that hydraulic traits of Juniperus species strongly reflect phylogenetic history. The high resistance to cavitation observed may help explain the survival of junipers during recent extreme droughts in the southwestern U.S. and their expansion into arid habitats across the western and central United States.