Groundwater is a fundamental yet frequently underappreciated factor in geotechnical design, with the capacity to significantly influence soil and rock behaviour, structural stability, and long-term performance of civil infrastructure. Inadequate consideration of groundwater conditions can lead to excessive deformation, instability, serviceability issues, and, in extreme cases, catastrophic failure. This paper examines the critical role of groundwater in geotechnical engineering through a series of five case studies drawn from Australian infrastructure and development projects. The case studies address a range of groundwater-related mechanisms, including slope instability driven by elevated pore pressures, impacts of construction dewatering on adjacent structures, cumulative effects of subterranean cut-off walls acting as flow barriers, collapse settlement in fill, and internal erosion manifested as sand boiling during flood events. The paper highlights the importance of understanding groundwater flow regimes, hydraulic connectivity, and soil–water interaction mechanisms at both site and regional scales. Collectively, the case studies reinforce the need for early hydrogeological assessment, appropriate monitoring, and integrated groundwater modelling as essential components of robust geotechnical design and risk management.