The chapter introduces the concept of plant apparency and chemical defense in the context of insect-plant interactions. It highlights the goal of insect ecology to understand the patterns of interaction between plants and herbivorous insects, both in ecological and evolutionary time. The chapter emphasizes the significance of secondary chemical compounds (natural products) in plants, which often serve as toxins to animals, microorganisms, and fungi. These compounds have led to a coevolutionary theory where plants evolved to produce these compounds in response to insect attacks, and insects evolved methods to tolerate these chemicals. This coevolutionary arms race involves plants deploying metabolic resources for defense, while insects invest energy in host location and attack mechanisms. The chapter also discusses how plant species can be seen as chemically defended "islands" that require chemical adaptation for insect colonization. Additionally, it notes that evolutionary changes in host plant ranges are more likely to occur among plants with similar secondary chemistry, and that chemical coevolution is not limited to insects but also affects other organisms, including mammals, reptiles, and fungi.The chapter introduces the concept of plant apparency and chemical defense in the context of insect-plant interactions. It highlights the goal of insect ecology to understand the patterns of interaction between plants and herbivorous insects, both in ecological and evolutionary time. The chapter emphasizes the significance of secondary chemical compounds (natural products) in plants, which often serve as toxins to animals, microorganisms, and fungi. These compounds have led to a coevolutionary theory where plants evolved to produce these compounds in response to insect attacks, and insects evolved methods to tolerate these chemicals. This coevolutionary arms race involves plants deploying metabolic resources for defense, while insects invest energy in host location and attack mechanisms. The chapter also discusses how plant species can be seen as chemically defended "islands" that require chemical adaptation for insect colonization. Additionally, it notes that evolutionary changes in host plant ranges are more likely to occur among plants with similar secondary chemistry, and that chemical coevolution is not limited to insects but also affects other organisms, including mammals, reptiles, and fungi.