The critical zone (CZ), extending from the vegetation canopy to unweathered bedrock, hosts coupled hydrologic, geochemical, and biological interactions that regulate soil health, water resources, and ecosystem sustainability. The subsurface CZ can extend tens of meters below the surface and is largely inaccessible, except in happenstance exposure from quarries or road cuts through mountain hillsides. Drilling and subsequent borehole sampling, monitoring and imaging reveal the importance of the deep subsurface for CZ evolution and function. However, drilling can be labor-intensive, require expensive, specialized equipment, and can only be done where the equipment can be deployed, limiting the number and placement of boreholes. To empower the next generation of critical zone scientists to employ drilling and downhole techniques, this review synthesizes emerging research objectives and methods commonly used during CZ drilling campaigns over the last 30 years. We focus on three CZ research themes: (1) physical and chemical weathering, (2) subsurface water storage and flow, and (3) solute, microbial, and gas dynamics. For each theme, we evaluate drilling techniques, sampling strategies, downhole logging approaches, long-term monitoring, and analytical methods that collectively enable diverse hypothesis-testing. We conclude by providing a vision for the future of drilling within the CZ, with a focus on novel drilling techniques aimed at recovering saprolitic material as well as borehole designs that can monitor and sample the vadose zone. Additionally, we emphasize that near-surface geophysics and data-model integration efforts are needed to expand borehole observations to the larger scales that are necessary to advance CZ science and inform ecosystem and water resource management.