Geology is essential for understanding the behavior of rock, soil, and other parameters such as rock mass, lithology, groundwater and hydrology, seismicity, faults, and climate for tunneling projects. These parameters help engineers design and select the methodology for tunnel excavation. Understanding the mechanical and physical behavior of soils, rocks, and rock mass together is critical for selecting conventional and mechanical excavation tunneling methods. This chapter briefly describes factors affecting tunneling performance, rock and soil properties, rock mass properties, major engineering rock mass classification systems, gas emission, underground water, testing methods and standards for using the mechanical miners, and understanding the rock mass squeezing characteristics.