X-ray computed tomography (CT) is an established technique for non-destructively characterizing geological cores. CT provides information on sediment structure, diagenetic alteration, fractures, flow channels and barriers, porosity and fluid-phase saturation. A portable CT imaging system has been developed specifically for imaging whole-round cores at the drilling site. The new system relies upon carefully designed radio-logical shielding to minimize the size and weight of the resulting instrument. Specialized X-ray beam collimators and filters maximize system sensitivity and performance. The system has been successfully deployed on the research vessel JOIDES Resolution for Ocean Drilling Program's legs 204 and 210, at the Ocean Drilling Program's refrigerated Gulf Coast Core Repository, as well as on the Hot Ice #1 drilling platform located near the Kuparuk Field, Alaska. A methodology for performing simple densiometry measurements, as well as scanning for gross structural features, is presented using radiographs from ODP Leg 204. Reconstructed CT images from Hot Ice #1 demonstrate the use of CT for discerning core textural features. To demonstrate the use of CT to quantitatively interpret dynamic processes, we calculate 95% confidence intervals for density changes occurring during a laboratory methane hydrate dissociation experiment. The field deployment of a CT represents a paradigm shift in core characterization, opening up the possibility for rapid systematic characterization of three-dimensional structural features, and leading to improved subsampling and core-processing procedures.