This paper describes an experimental and an analytical and numerical investigation into the buckling behaviour of cylindrical composite tubes under external hydrostatic pressure. The investigations concentrated on fibre reinforced plastic tube specimens made from a mixture of three carbon and two E-glass fibre layers. The lay-up was 0°/90°/0°/90°/0; the carbon fibres were laid lengthwise (0°) and the E-glass fibres circumferentially (90°). The theoretical investigations were carried out using a simple solution for isotropic materials, namely a well-known formula by “von Mises” and also by finite element analyses using ANSYS. The experimental investigations showed that the composite specimens behaved similarly to isotropic materials tested by various other researchers. The specimens failed by the common modes associated with this study, namely due to elastic buckling, inelastic buckling and axisymmetric yield failure. Furthermore it was discovered that the specimens failed at changes of the composite lay-up due to the manufacturing process of these specimens. These changes seem to be the weak points of the specimens. For the theoretical investigations two different types of material properties were used to analyse the composite. These were calculated properties derived from the properties of the single layers given by the manufacturer and experimentally obtained properties. Two different approaches were chosen for the investigation of the theoretical buckling pressure, a program called “MisesNP”, based on a well-known formula by von Mises for single layer isotropic materials, and two finite element analyses using the famous computer package called “ANSYS”. This latter analyses simulated the composite with a single layer orthotropic element (Shell93) and also with a multi layer element (Shell99). It was found out that the results obtained with ANSYS predicted questionable buckling pressures that could not be reproduced logically. Nevertheless this report provides Design Charts for all approaches and material types. These Design Charts allow the possibility of obtaining a ‘plastic knockdown factor’. The theoretical buckling pressures obtained using MisesNP or ANSYS can then be divided by the plastic knockdown factor, to give predicted buckling pressures. This method can be used for the design of full-scale vessels.