One of the most interesting chapters in the creative work of Leonardo da Vinci concerns his research on the problem of human flight using artificial wings. Leonardo was the first scientist to study this subject seriously, and his manuscripts contain a large number of drawings and short descriptions of diverse flying apparatuses. He continually returned to this theme throughout his entire working life: his first projects on flying machines date from the 1480s, and his last from the 1510s. This essay analyzes the development of da Vinci’s ideas about human flight and comes to the following conclusion: the idea of the airplane did not originate sui generis, as most historians of aviation believe, but evolved from designs for birdlike flying machines with flapping wings (ornithopters) through a series of intermediate halfplane, half-ornithopter hybrids.