Operating in the wavelength range 5-28 mu m, the Mid-infrared Array eXpandable (MAX) camera contains a 128x128 Si:As Blocked Impurity Band (BIB) detector tailored for high background, low noise performance. The term "expandable" refers to our ability to substitute one of the newer generation 256x256 arrays, when they become available. The all-reflective optics provide diffraction limited performance, and a large, interchangeable filter set allows both broad and narrow band imagery. MAX was built to our specifications by Infrared Laboratories of Tucson, and it achieved first light on UKIRT in November 1995. In this paper, we present the design and real-world performance of MAX, together with a sample of the scientific results already published.