Competitor, stress-tolerator, ruderal (CSR) theory, regarded as one of the best developed plant ecological strategy schemes, mainly studies the adaptive specialisation that plants evolve to achieve fitness in response to natural selection pressures, resulting in a triangular distribution of species optima along axes of habitat productivity and disturbance. From the perspective of functional ecology, the CSR theory reflects plant ecological strategies by explaining the principal functional spectra in the multidimensional trait space, that is, the world-wide economics spectrum and the size spectrum as fundamental gradients of plant evolution are major components of the CSR strategy variation. The ecological strategies adopted by plants then determine their ecological niche in the community and ultimately affect ecosystem processes and functions. Thus the CSR theory could not only be used as a framework for functionally classifying plants and predicting impacts of environmental change on vegetation, but also serves as a conceptual framework for investigating and interpreting community processes and ecosystem functioning. Since the concept of CSR strategies was first raised by Grime in 1974, the idea of CSR strategies has attracted much attention from ecologists, and the CSR theory has formed a relatively complete system of theories and research methods over the last half century. In this paper, we introduce the development and characteristics of the CSR theory and its methods to expound the core philosophy of the theory, review the verification and application of the CSR theory and its models by addressing the relevance of CSR strategies to functional trait variations, species adaptation, community processes and ecosystem properties, and summarize the advances in this theory. The CSR theory has been extensively validated by many studies and widely applied as a very useful analysis tool to plant ecology research on a variety of scales, but it needs further research, such as the missing fourth corner in the Grime's C-S-R triangular model, the rest functional variations beyond the principally functional trait spectra that are not explained by the CSR scheme, and the relationships between CSR strategies and the intraspecific trait variation and phylogenetic signal. Especially for the rest functional variations, the CSR theory does not preclude the existence of additional axes of variability subordinate to the principal functional spectra, nor that these may be certainly key to fitness and survival in specific cases. The CSR theory could be seen as a general adaptive strategy, since it is consistent with economics and size tradeoffs evident throughout the tree of life, hinting at overarching natural selection filters operating universally within the biosphere.