[3] His doctoral students include Alan Ross Anderson, Ruth Barcan Marcus, and William W. Tait.
Fitch was the inventor of the Fitch-style calculus for arranging formal logical proofs as diagrams.
[4] In his 1963 published paper "A Logical Analysis of Some Value Concepts" he proves "Theorem 5" (originally by Alonzo Church), which later became famous in context of the knowability paradox.
[5] Fitch worked primarily in combinatory logic, authoring an undergraduate-level textbook on the subject (1974), but he also made significant contributions to intuitionism and modal logic.
He was interested in the problem of the consistency, completeness, categoricity, and constructivity of logical theories, especially nonclassical logics, and contributed to the foundations of mathematics and to inductive probability.