Karl Wilhelm Otto Lilienthal (23 May 1848 – 10 August 1896) was a German pioneer of aviation who became known as the "flying man". He was the first person to make well-documented, repeated, successful flights with gliders, therefore making the idea of "heavier than air" a reality. Newspapers and magazines published photographs of Lilienthal gliding, favourably influencing public and scientific opinion about the possibility of flying machines becoming practical.

Lilienthal's work led to his developing the concept of the modern wing. His flight attempts in 1891 are seen as the beginning of human flight and the "Lilienthal Normalsegelapparat" is considered to be the first airplane in series production, making the Maschinenfabrik Otto Lilienthal in Berlin the first air plane production company in the world.

Otto Lilienthal is often referred to as either the "father of aviation" or "father of flight".

On 9 August 1896, his glider stalled and he was unable to regain control. Falling from about 15 m (50 ft), he broke his neck and died the next day.