The Harwell system for measurement of mg size samples, using the Brookhaven miniature gas counters comprises 12 counters, of two different sizes, which are operated simultaneously within a single NaI crystal acting as an anti-coincidence guard counter. Brief details will be given of the construction and commissioning of the system as a whole, including the counter assembly, shield design, electronics, data capture, data analysis and chemical processing and filling procedures. The performance of the system is discussed and an overall view of the fields of application for which the counters have important applications is given. Some examples are given of specific samples measured and the systems potential.