Penetration, in the context of cement and concrete science, refers to the process by which external substances, such as water, chemicals, and gases, infiltrate the material’s microstructure. This process is critical to understanding durability and longevity in harsh environmental conditions. When external agents penetrate concrete, they can initiate processes like corrosion of reinforcing steel, freeze-thaw cycles, and chemical degradation, notably from sulfates and chlorides. Mechanistically, penetration occurs through capillary action, microcracks, or both, exacerbated by the presence of voids and the concrete’s inherent permeability. The effects on concrete are predominantly negative, leading to a reduced service life, loss of structural integrity, and increased maintenance costs. However, understanding these processes also facilitates the development of more durable composite materials, incorporating additives and treatments that reduce permeability and enhance resistance to environmental stressors.