The object of this monograph is to document what is most interesting about linear monoids. We show how these results ?t together into a coherent blend of semigroup theory, groups with BN-pair, representation theory, convex - ometry and algebraicgrouptheory.The intended reader is one who is familiar with some of these topics, and is willing to learn about the others. The intention of the author is to convince the reader that reductive monoids are among the darlings of algebra. We do this by systematically assembling many of the major known results with many proofs,examples and explanations. To further entice the reader, we have included many exercises. The theory of linear algebraic monoids is quite recent, originating around 1980. Both Mohan Putcha and the author began the systematic study in- pendently. But this development would not have been possible without the pioneering work of Chevalley, Borel and Tits on algebraic groups. Also, there is the related, but more general theory of spherical embeddings, developed largely by Brion, Luna and Vust. These theories were developed somewhat independently, but it is always a good idea to interpret monoid results in the combinatorial apparatus of spherical embeddings. Each chapter of this monograph is focussed on one or more of the major themes of the subject. These are: classi?cation, orbits, geometry, represen- tions, universal constructions and combinatorics. There is an inherent div- sity and richness in the subject that usually rewards a stalwart investigation.