The BGS Hydrothermal Laboratory enables the study of chemical reactions between fluids and rocks under conditions found in the top few kilometres of the Earth's crust. In its 30+ years, it has been at the centre of numerous investigations that require well-controlled conditions to study reaction processes under in-situ conditions (i.e. elevated temperatures and pressures), and it is probably unique in the range of very different studies that have been investigated in the laboratory. Static (batch) and flow-through equipment are available in the laboratory, with useable volumes ranging from less than one millilitre to over ten litres. Much of the equipment can withstand high temperatures and pressures, with current standard operating conditions up to about 400°C and 500 bar. Although some equipment is 'off the shelf', numerous pieces are novel, having been specially developed for the laboratory. The experimental reaction products are characterised using a wide range of fluid chemical and mineralogical analytical techniques that are available within other dedicated laboratories at the BGS.