In discussing agencies for the improvement of teachers in service, one's mind naturally turns first to those periodical meetings of teachers, such as institutes, that are established by law. The data pertaining to meetings of this nature that have been collected are summarized in this paper. These were gathered primarily from State and Territorial school laws, but this source was liberally supplemented by State educational reports, by institute and summer-school bulletins, and by correspondence with State school officers. This monograph is intended to present what is actually being done, and not merely to give an outline of the legal provisions. The word "institute" has not a very, definite meaning in educational literature. It is a blanket word that is applied indiscriminately to any officially established gathering of teachers. Because of this fact, these gatherings have been divided, so far as possible, into classes according to their nature, regardless of the names used in the laws to designate them. On this basis three classes have been made, as follows: (1) Teachers' institutes; (2) summer normal or summer training schools; and (3) teachers' meetings. Perhaps a fourth class--teachers' associations--should have been added, but the legally established gathering in only one State--Maine--approaches the nature of the customary teachers' association. The criteria that distinguish institutes, summer normal schools, teachers' meetings, and teachers' associations are discussed in this paper. An index is included. Individual sections contain footnotes. [Best copy available has been provided.]