A modular gas control device for use with a compressed gas cylinder (111) comprises a primary module (152) and a secondary module (252) mounted on the primary module. The primary module comprises a first supporting body (154) having a first main gas flow path (155) through the body. The supporting body has input connecting means (156) for mounting the body on the cylinder (111) and connecting the gas flow path (155) to communicate with the gas cylinder through a first flow path (157). Pressure reducing means (166) provides gas in the flow path at a lower pressure than in the container. Output connecting means (170) downstream of the pressure reducing means provides a low pressure outlet from the main gas flow path. A high pressure shut off valve (164) is positioned upstream of the pressure reducing means, and filling means (161, 160) allows filling of the cylinder with compressed gas through the input connecting means (156) along a second flow path (159) separate from the input flow path (157). The secondary module (252) has a corresponding supporting body (254) and main flow path (255) and corresponding output connecting means (270) and corresponding input connecting means (256) for mounting the secondary module (252) on the primary module (152). The supporting body (254) of the secondary module has a combination of two or more functional components comprising means for measuring and/or varying parameters of gas flow in the second supporting body, and/or for switching and/or venting and/or mixing gas flow in the second supporting body.