Plant functional traits play a crucial role in linking morphology, structure, and function of plants, making their variation, adaptations, and optimization mechanisms in response to changing environments long-term hot topics in ecology. Research on plant functional traits has recently expanded from small-scale, single-organ, and individual functional traits to large-scale, multiorgan, and multifunctional traits, thereby improving the development of biogeography, ecosystem ecology, and global change ecology. In this chapter, we discuss the research progress on plant functional traits at the organ level to connect with other chapters and develop a continuous understanding from the organ to the ecosystem level. First, as the primary organ responsible for plant energy production through photosynthesis, leaf functional traits may reflect photosynthetic capacity, resource utilization strategy, and adaptability of plants to the external environment. In recent decades, numerous studies on leaf functional traits have examined the spatiotemporal variations in morphological traits, functional elements, anatomical traits, vein and stomatal traits, and the key influencing factors. The main factors influencing the spatiotemporal variation in leaf functional traits are species-specific differences, soil conditions, and climate. Furthermore, researchers have extensively investigated the geographical patterns of leaf functional elements, studied the stoichiometric properties of functional elements and their responses to global change factors, and proposed relevant theoretical hypotheses, such as the stability of limiting elements hypothesis, temperature–plant physiology hypothesis, biogeochemistry hypothesis, and soil matrix age hypothesis. Studies on root functional traits have lagged their aboveground counterparts, such as leaves. Currently, root ecology focuses on the adaptive strategies of root morphological traits, functional elements, and mycorrhizal traits to the fluctuating environment, multiple dimensions of trait relationships, and the underlying influence on ecosystem processes and functions. Although related research has expanded to all aspects of ecology and promoted the rapid development of plant ecology, there is considerable scope for theoretical advances. For example, there is an urgent need to explore the synergies of multiple organs or functional traits, the effects of intraspecific variation and phylogeny on plant functional traits, and the adaptive mechanisms of plant functional traits under extreme environmental conditions. Furthermore, organ-level functional trait studies should be combined with these control experiments to effectively reveal the plant response mechanism to changing environmental conditions.