This report details the concept definition of using the External Tank (ET) of the Space Shuttle as the basis for constructing a large-area gamma-ray imaging telescope in space. The telescope will be used to locate and study cosmic sources of gamma-rays of energy greater than 100 MeV. The report describes in detail both the telescope properties and the means whereby an ET is used for this purpose. A parallel is drawn between those systems that would be common to both a Space Station and this ET application. In addition, those systems necessary for support of the telescope can form the basis for using the ET as part of the Space Station. The major conclusions of this concept definition are that the ET is ideal for making into a gamma-ray telescope, and that this telescope will provide the substantial increase in collecting area recommended by the National Academy of Sciences. The principal recommendation is that NASA proceed at this time with a design definition of the concept presented herein.