Understanding the pump-down characteristics of a large vacuum vessel is critical for determining whether the vacuum system is behaving normally, or if it has a source of outgassing or developed a real leak. Finding leaks on large vacuum chambers using a helium leak detector is fast, efficient, and cost-effective. The proper selection of a leak detector, the connection of the detector to the vacuum system, and the appropriate use of helium tracer gas are fundamental to a successful leak test. Typically, large vacuum chambers use a high vacuum pump (turbo or diffusion) backed by a roots-type blower and mechanical pump. This method provides the fastest response and cleanup. While this Technical Overview describes large systems that are pumped in this manner, the methods described are applicable to vacuum chambers of all sizes.