A theoretical analysis has been made of the effect of gradient, wind, and combinations thereof, on the take-off and landing of aeroplanes. Formulas and charts are given which permit the determination of the length of the ground run for any given conditions providing that the performance on a level surface in still air is known.The effect of wind and current on the take-off of seaplanes has also been investigated. Formulas and charts have been derived for the determination of take-off time and distance under any given conditions from a knowledge of the take-off speed, time, and distance in still air and still water. This analysis indicates that in the take-off of seaplanes the wind is in general of more importance than the current, and, in consequence, take-off should always be carried out upwind.