Abstract: Cementitious composites are the most widely used engineering materials. Owing to their inherent characteristics and the complex service environment, infrastructure constructed with such composites faces severe challenges regarding safety, service life, toughness, and sustainability (e.g., low resource consumption and low carbon emissions. Nanoscience and technology can be used to understand and regulate cementitious composites at the most fundamental level, which significantly enhances their mechanical properties and durability, endows them with functional characteristics, and thereby enables these composites to achieve (ultra-)high performance, multi-functional and smart properties, and low carbon emissions, providing effective solutions to the aforementioned challenges. However, the mechanisms of nano-engineering for cementitious composites have not yet been fully clarified, systematically established, or unified. Consequently, the concept of the “nano-core effect” is proposed to reveal the intrinsic mechanisms of nano-engineered cementitious composites. This effect consists of two main aspects: the nano effect and the core effect. The nano-effect refers to the small particle size (at the content level of 0.1vol%, the number of nanofillers in 1 cm³ of cementitious composites can reach up to 10¹⁸) and high specific surface area (reaching up to 1000 m²·g⁻¹, which is far higher than that of submicron silica fume (≈150 m²·g⁻¹) and micrometer-sized cement particles (≈0.35 m²·g⁻¹)). The core effect includes adsorption effect, filling effect, bonding effect, and intelligent/functional effect. The "nano-core effect" can induce the assembly, growth and distribution of surrounding ions and hydration products (e.g., nano-engineering can induce the formation of super-hard calcium silicate hydrate gel phase with a hardness of up to 2.50 GPa; meanwhile, it can also refine the nano-scale interlayer and gel pores and induce pore transformation, thus leading to the shrinkage and densification of calcium silicate hydrate gel structure). In addition, it forms a widely distributed reinforcement/modification network based on the intrinsic properties of nanofillers, thereby improving the internal structure and performance of composite materials. This paper systematically expounds the basis for the proposal of the "nano-core effect", its mechanism of action, and its key influencing factors. Additionally, it introduces the concepts of "nano-core effect control zone" and "nano-core-shell unit" to establish the connection between the nano-micro-fine-macro multi-scale structure and performance of nano-engineered cementitious composites. This paper aims to lay a theoretical foundation for guiding the design, preparation, and application of nano-engineered cementitious composites.