Fieldwork is one of the primary methods for studying medicinal plants and materials, and information thus obtained can be valuable for experiments performed in the laboratory. Meanwhile, results of experiments in the laboratory can be brought back to the field for verification and further investigation. A combination of field and laboratory work has led to effective progress in studies of medicinal plants in the field of pharmacognosy. However, the collection of samples with information through fieldwork is not easy, and it fundamentally requires a great deal of research experience. Geographical, ethnical, and political affairs often affect its performance, and to establish a good cooperative relationship with foreign localities is inevitably required. Beyond these difficulties, fieldwork can provide a framework for the research project and excellent and unique viewpoints concerning the target. This review article describes studies on perilla, agarwood, and cinnamon, focusing mainly on the results of fieldwork performed in Indochina on these species. All three of these medicinal plants contain essential oils, and their composition varieties, biosynthetic pathways, pharmacological activities, or induction mechanisms for production are principally investigated through shuttling between fieldwork and laboratory experiments.