Rock cores provide essential data for the exploration, evaluation and production of oil and gas reservoirs. Physical rock samples allow geoscientists to examine firsthand the depositional sequences penetrated by a bit and offer direct evidence of the presence, distribution and deliverability of hydrocarbons. Cores provide ground truth for calibration of well logs and can reveal variations in reservoir properties that might be undetectable through downhole logging measurements alone. This Defining Series article sets out the methods of acquiring cores—using drill pipe and wireline—and their analysis and role in the industry. The Oilfield Review Defining Series provides E&P professionals with concise, authoritative, up-to-date summaries of a wide range of industry topics.