The Baker Street Irregulars is a literary society dedicated to the study of Sherlock Holmes, Dr. Watson, Sir Arthur Conan Doyle, and the Victorian world. Established in 1934, it holds the honor of being the oldest Sherlockian society globally.

The BSI publishes a wide range of scholarly works focused on Sherlock Holmes and hosts various events, including the annual BSI Weekend held every January. As an international literary society, the BSI is joined by local groups known as BSI Scion Societies, with membership determined by each individual group. While the BSI membership is exclusive and available by invitation only, individuals can still subscribe to the Baker Street Journal, purchase BSI Books, and attend most BSI Events without being a member.

The BSI also actively engages in preserving its rich history through the BSI Trust, which collects and opens up access to materials related to the society. Notably, the BSI is a non-profit organization exempt under Section 501(c)(3) of the Internal Revenue Code