 The algebra of logic, as an explicit algebraic system showing the underlying mathematical structure of logic, was introduced by George Boole (1815–1864) in his book The Mathematical Analysis of Logic (1847). It is therefore to be distinguished from the more general approach of algebraic logic.  The methodology initiated by Boole was successfully continued in the 19th century in the work of William Stanley Jevons (1835–1882), Charles Sanders Peirce (1839–1914), Ernst Schröder (1841–1902), among many others, thereby establishing a tradition in (mathematical) logic. From Boole’s first book until the influence after WWI of the monumental work Principia Mathematica (1910–1913) by Alfred North Whitehead (1861–1947) and Bertrand Russell (1872–1970), versions of the algebra of logic were the most developed form of mathematical logic above all as presented in Schröder’s three volumes Vorlesungen über die Algebra der Logik (1890–1905). Furthermore, this tradition motivated the investigations of Leopold Löwenheim (1878–1957) that eventually gave rise to model theory. In addition, in 1941, Alfred Tarski (1901–1983) in his paper “On the calculus of relations” returned to Peirce’s relation algebra as presented in Schröder’s Algebra der Logik. The tradition of the algebra of logic played a key role in the notion of Logic as Calculus as opposed to the notion of Logic as Universal Language. Beyond Tarski’s algebra of relations, the influence of the algebraic tradition in logic can be found in other mathematical theories, such as category theory. However this influence lies outside the scope of this entry, which is divided into 10 sections.