Logtk : A Logic ToolKit for Automated Reasoning and its Implementation. We describe the design and implementation of Logtk, an OCaml library for writing automated reasoning tools that deal with (possibly typed) first-order logic. The library provides data structures to represent terms, formulas and substitutions, and algorithms to index terms, unify them. It also contains parsers and a few tools to demonstrate its use. It is the basis of a full-fledged superposition prover.