Criterion and caveats of sampling design vary depending on the hydrocarbon type, reservoir heterogeneity, development maturity, and cost. Fluid systems are interpreted to be in hydrodynamic equilibrium; that is, fluid properties follow the same distribution bounded only by changes in pressure and temperature across the reservoir. Reservoir hydrocarbons are interpreted to be either saturated or undersaturated. Conceptual understanding the hydrocarbon system/s of the reservoir/s is a key to a successful sampling program. Pressure data provide an insightful appreciation of the compartmentalization and compositional changes in the reservoir, making it fundamental for hydrocarbon sampling design. Gravity segregation models allow for an easy calculation of the compositional variation by accounting the changes of chemical potential of each of the hydrocarbon components. The main objective of sampling is to collect the designed volume of in situ hydrocarbon fluid with a composition representative of that of the reservoir without any contaminants.