The first flora of Illinois was prepared in 1846 by Dr. Samuel B. Mead, a physician from Augusta, Illinois. Between 1846 and 1963, several people published plant lists and floras for various parts of Illinois. In 1975, Robert H. Mohlenbrock published his first edition of Guide to the Vascular Flora of Illinois, followed by a second expanded and revised edition in 1986. This latest Vascular Flora of Illinois includes over thirty-two hundred species, an increase of almost three hundred since 1986. In addition, for the first time, taxa below the rank of species are recognized and may be identified by keys. Investigating seldom-visited patches of prairies, wetlands, and forested canyons, Mohlenbrock has added several native species to this flora. And while there has been extensive exploration for plants, Mohlenbrock has also researched the herbaria in an attempt to verify previous reports of records of Illinois plants. Because of a reinterpretation of existing genera, the number of genera of Illinois plants has increased markedly. Recent biosystematic techniques have begun to substantiate the genera that botanists such as John Kunkel Small, Per Axel Rydberg, and Edmund C. Greene proposed nearly a century ago. The sequence of groups in this book is ferns, conifers, and flowering plants, with dicotyledons given before monocotyledons. Within each group, the families are arranged alphabetically, as are the genera within each family and the species within each genus. For each taxon recognized in this book, Mohlenbrock gives a common name if one is generally used in Illinois. He follows this by an indication of flowering time for flowering plants and of spore-production time for ferns and their relatives. He also provides a habitat statement and a general comment on distribution in Illinois for each taxon. Synonyms for some other scientific names used previously for a taxon appear in italics. This book contains indexes both for common names and for family and genus names.