Shipboard measurements at the time of loading and discharging of petroleum cargoes form an integral part of hydrocarbon management. The quantities as determined by vessel measurements are not only significant for analysis and reconciliation purposes, but are also used fairly frequently as the basis for the bill of lading (BOL) or the official custody transfer quantity when availability of measurements elsewhere is limited or where contracts specify this approach. This article discusses some of the technical issues involved and provides recommendations wich are aimed at assisting and improving the accuracy of measurement and sampling on ships.