A fluid vessel insertion device (10), typically for medical use, comprises a puncture member (18) arranged for locaton within a sleeve (20), and a retraction mechanism (44, 48, 60) for retracting the puncture member relative to the sleeve. The puncture member (18) is retractable in use between a first condition in which an end (18A) of the puncture member protrudes beyond an end (24) of the sleeve and a second condition in which the end of the puncture member is within the sleeve (20). The retraction mechanism (44, 48, 60) actuates the puncture member between the first and second conditions in response to fluid pressure at the end (18A) of the puncture member. The device may be used during positioning of the sleeve in a desired fluid vessel, such as, for example, during intravenous cannulation.