INTRODUCTION An excavation in the near-surface underground introduces steep hydraulic energy and chemical gradients to a zone of variable extent around the excavation perime­ ter. These construction-induced gradients overcome existing gradients to draw gases toward the excavation. This chapter focuses on the forces that control gas inflow to tunnels and shafts during construction. These principles of gas inflow can be applied to other types of excavations and completed structures in the near­ surface underground.