This volume presents three chapters discussing a range of topics. Chapter 1 deals with the development of efficient methods for compound dereplication that have been critical in the re-emergence of research on natural products as a source of new drug leads. It describes the main methods of dereplication, which rely on the combined use of large natural product databases and spectral libraries, alongside the information obtained from chromatographic, UV-Vis, MS, and NMR spectroscopic analyses of the samples of interest. Chapter 2 describes 989 plant natural products and their ecological functions in plant-herbivore, plant-microorganism, and plant-plant interactions. These compounds include alkaloids, phenols, terpenoids, and other structural types. The information presented should provide the basis for in-depth research on these plant natural products and their natural functions, and also for their further development and utilization. Chapter 3 focuses on lichens, with each constituting a symbiotic association composed of a primary mycobiont and one or more photobionts living mutualistically. Covered are lichens and their bionts, taxonomic identification, and their chemical constituents as exemplified by what is found in lichen biomes, especially those endemic to North America. Extraction and isolation procedures, as well as updates on dereplication methods using mass spectrometric GNPS and NMR spectroscopic spin network fingerprint procedures, and marker-based techniques to identify lichens are discussed. The isolation and structure elucidation of secondary metabolites of an endolichenic Penicillium species that produces bioactive compounds is described in detail.