There are many types of Motorcycle Modification ranging from minor adjustments to suit the riders physique or preferences to creation of almost completely new machines with very little of the original remaining. In between there are many possibilities including changes to the bikes appearance to differentiate it from the mass of identical production machines, changes to improve performance, handling, or reliability, changes to classic bikes to make them more usable in modern road conditions, or just attempts to create the modifiers vision of his perfect bike. All these types of modification have one thing in common, the overriding need to ensure that the change does not make the machine less safe to ride than the original. This book proposes a simple method, based on long established aircraft and aero engine practices, of reviewing any intended modification at an early stage so that any possible safety problems can be avoided. The book is more than just a basic manual and incorporates numerous sketches and photographs to illustrate various points together with many memories from a long career, various related and unrelated thoughts and a few original and possibly controversial suggestions which occurred to the author at the time of writing. It invites the reader to either agree or disagree with any or all of these deviations from the basic principles of the method. There is also an attempt to predict the future of motorcycle development and modification. The book avoids much engineering theory to ensure that it is readily understandable to anyone who does not claim to be an engineer or contemplate modifying a motorcycle but is just interested in all things mechanical. It may also be useful to young engineers embarking on an interesting and rewarding career by offering many insights into the engineering world which are not usually included in engineering courses.