Ryū (流) is synonymous with school of a Japanese martial art.

Although there may be many ryūha (derivations) based on a ryū of a given martial art, each will be unique and have its own views concerning the martial art.

Many swordsmanship and martial arts techniques were passed down from the creators of the techniques to their offspring, and to any deshi (student) which wanted to learn the technique. The largest schools had their own dōjōs and many students.

Swordsmen had to study the techniques from the various schools so they would know how to block them in combat. Many ryū also had secret techniques, only the students of the school were allowed to learn.