The aim of this book is to survey a number of chemical compounds that some chemists, theoretical and experimental, find fascinating. Some of these compounds, like planar carbon species or oxirene, offer no obvious practical applications; nitrogen oligomers and polymers, in contrast, have been touted as possible high-energy-density materials. What unites this otherwise eclectic collection is that these substances are unknown and offer a challenge to theory and to synthesis. That such a challenge exists is in some cases almost obvious to most chemists. The instability of nitrogen polymers, for example, might be taken nearly as an axiom, to be quantified, but not refuted by computations and to be subjected to an almost superfluous (but rather challenging) validation by synthesis. On the other hand, oxirene, the unsaturated relative of the prosaic oxirane, presents no immediately obvious oddity, yet this molecule has defied all attempts at synthesis and remains a theoretical conundrum, in that it is not certain if it can even exist. It is hoped that this collection of idiosyncratic molecules will appeal to chemists who find the study of chemical oddities interesting and, on occasion, even rewarding. 'A great romp through imagined molecules, a challenge to the talents of synthetic chemist! Errol Lewars leads us expertly through a wonderland of the chemical imagination, fascinating molecular structures that do not (yet) exist!' - Prof. Roald Hoffmann - Nobel Laureate, Chem. 1981- Cornell University, New York, USA. 'This book is an educational and enjoyable read, devoted to species on the fringes of chemical, calculation and conceptual plausibility' - Prof. Joel Liebman, University of Maryland, Baltimore County, USA.