Tryptophan is not only an essential amino acid but also an effective signaling molecule as an indicator of amino acid sufficiency. Serotonin is biosynthesized from the essential amino acid tryptophan. The first chapter of this book deals with the effects of tryptophan as well as some strategies of treatments that modify serotonin levels on cognitive processes during aging and neurodegenerative diseases. The changes of tryptophan metabolism under immunological challenges and physiological conditions like stress are examined as well. In the third chapter, the authors illustrate the key role of L-tryptophan in metabolic functions with special emphasis on the alterations of circadian rhythms. The remaining chapters of the book focus on new research findings on L-tryptophan synthesis from D-serine; the complex experimental and theoretical studies of the tryptophan molecule in its collisions with low and high energy electrons; and finally, a commentary on a novel tryptophan-selective amino acid transporter that is functionally coupled to ID01-dependent signaling pathways.