Abstract: This invention relates to a grab sampler for liquids especially for crude oil flowing through pipelines. The sampler has a set of concentric tubular members including an outer body with an inlet and outlet, and a sleeve and a hollow piston which are respectively axially slideable within the body and the sleeve. The piston has an internal non-return valve as its base. The sleeve, the piston base and the body are capable of defining a sample chamber so as to trap a sample of the liquid flowing through the pipeline. Application of pressure on the piston displaces the trapped sample through the nonreturn valve to the inside of the piston and is recovered therefrom by sample removal means.