Al-Zn binary alloys manifest various dendritic growth modes. In the fcc alloys (up to 82% Zn), primary dendrites grow in <100> for compositions below about 25%Zn , <110> above 55%Zn, and display seaweed patterns at intermediate compositions. Compositions above 82%Zn have a hexagonal primary phase, with preferred primary growth directions in <10<U>1</U>0> and <0001>. These alloys also display an orientation transition, where the <10<U>1</U>0> orientations are preferred at low growth rates, but at higher growth rates the <0001> orientation becomes more prevalent. Solidification experiments, X-ray tomography and phase-field computations, performed in collaboration with Michel Rappaz, show that the orientation transition in fcc alloys can be explained by variation in solid-liquid surface energy anisotropy associated with Zn content of the alloy, whereas in the hexagonal alloys, the transition is caused by differences in the dynamics of tip growth, resulting from the presence of forbidden orientations in the basal plane.