The present book investigates the spacing and orientation selection in directional solidification of Aluminum and Magnesium alloys under transient growth conditions, which represent the conditions dominant in industrial casting. Dendritic spacing and morphology established during casting often sets the scales of the downstream microstructure during manufacturing of alloys. This is particularly true in emerging technologies such as twin belt casting. The mechanical and physical properties of alloys are thus highly dependent on the morphology established during solidification. It is shown both experimentally and through phase field modeling that the previously obtained geometrical models to predict the dendrite spacing is not valid under transient growth. Instead, the initial conditions and history of the growth highly influence the final spacing. The orientation selection of hexagonal dendrites is discussed in chapter four.