Aiki, the unbalancing element of the martial arts of Aikido and Aikijujutsu, is the least understood aspect of these arts. Generally, descriptions of Aiki vary from the mystical to vague concepts like blending with the attacker's energy. This book gives a meaningful and practical bio-mechanical definition of Aiki. This description of Aiki clearly differentiates it from the unbalancing methods used in the associated martial arts of Judo, Jujutsu and Wrestling. Using this definition, Aiki can be applied in three fundamental forms. For each form, a detailed description of how Aiki is applied is given and is supplemented with demonstrations, so that the reader can experience the unbalancing effect of Aiki. Examples are described in detail, showing how each form of Aiki is applied in the techniques of Aikido and Aikijujutsu. The explanation of Aiki given here unifies the arts of Aikido and Aikijujutsu. Throughout the book, the ideas discussed are supported with illustrated diagrams. The Principle of Aiki described in this book has been used to analyse the unbalancing element of Aikido and Aikijujutsu techniques recorded on Youtube by well known Aikido and Aikijujutsu Sensei. This supporting material is available on the author's website aiki.simdif.com.