To attempt to discuss aerodynamic theory in a single lecture is perhaps a rather rash undertaking, for the subject is so vast that a proper discussion would require a volume, or even a series of volumes. My object, however, is not to cover the whole field of aerodynamic theory with its manifold applications to aeronautical engineering, but rather to discuss some of the fundamental conceptions on which the theory rests, and to discover where theory can be helpful and where it is necessary to rely wholly on experimental results.