Terpenoids are the secondary metabolites of plant derivatives, mainly found in essential oils derived from higher plant sources. Terpenes are simple hydrocarbons, whereas terpenoids are modified terpenes containing different functional groups and oxidized methyl groups. This volume includes seven chapters examining topics related to terpenes and terpenoids. Chapter One describes mono- and sesquiterpenoids isolated from plants of the Inula L. genus and discusses biological activity of extracts and individual sesquiterpene lactones. Chapter Two covers the nature of terpenoids together with their biological activities and their isolation from endemic species from the Asteraceae family growing in Algerian Sahara and used as medicinal plants. Chapter Three introduces a catalyst system using Keggin heteropolyacid salts and hydrogen peroxide to epoxidize terpenic alcohols. Chapter Four examines the phyto-active components of terpenoids as sources of insecticide. Chapter Five describes how organic compounds containing functional groups like ketone, alcohol, and epoxide undergo rearrangement with sulfuric acid, yielding unexpected results. Chapter Six focuses on alkoxylation of terpenes over heterogenous catalysts. Lastly, Chapter Seven presents a discussion about the importance of Brazilian flora for herbal medicine.