Book excerpt: Cyclobutadiene and Related Compounds describes the four-membered carbocyclic compounds that have only trigonally hybridized carbon atoms in the ring. The book discusses the chemistry of cyclobutadiene and the substituted cyclobutadienes, including the preparation of cyclobutadiene-metal complexes that can be done in four ways. These are (1) by reaction of acetylenes with transition metal compounds, (2) from cyclobutenyl metal complexes, (3) by transformation of other cyclobutadiene-metal complexes, and (4) by trapping cyclobutadienes with transitional metal compounds. The text also describes cyclobutadiene dianion, the chemistry of the cyclobutadienequinones, the condensed methylenecyclobutene aromatic systems, and the higher aromatic analogs of benzocyclobutadiene. The book cites proofs of the structure of biphenylene using chemical and physical methods. The text also presents theoretical aspects of the cyclobutadiene problem including studies regarding the ?-electronic state of cyclobutadiene; the comparisons of some of these studies; the framework energies and Janh-Teller considerations; and the substituted cyclobutadienes, cyclobutadiene divalent ions, and metal complexes. This book is suitable for scientists and researchers working in the field of organic chemistry, analytical chemistry, and molecular physics.