This article provides descriptions of four new science and technology encyclopedias that are appropriate for inclusion in upper elementary and/or middle school reference collections. "The Macmillan Encyclopedia of Weather" (Stern, Macmillan Reference/Gale), a one-volume encyclopedia for upper elementary and middle level students, is a well-constructed reference book that provides a good introduction to facts about weather and climatology. "The Encyclopedia of North American Trees" (Benvie, Firefly Books) has descriptions and illustrations of the species of trees, and includes information on distribution, soil types, moisture and temperature requirements, growth, planting, and the use of trees by birds, other animals, and man. "The Illustrated Encyclopedia of Knots" (Budworth, Lyons Press) provides information on the history of knots; materials, terms, and tips for knot-tying; a Knot Finder Table of Contents; and step-by-step illustrations for typing different kinds of knots. "The Encyclopedia of Mammals" (Macdonald, Facts on File) consists of three volumes, arranged by classification. Volume One includes carnivores and sea mammals; Volume 2 has primates and large herbivores; Volume 3 covers small herbivores, insect eaters, and marsupials. This is an enlarged and revised version of the 1984 edition and a comprehensive resource that would be useful in middle school library media centers to supplement the more elementary animal encyclopedias.