Project SUE is writing a complete operating system for the IBM System/360 family of computers in a language designed especially for that purpose. The System Language was designed from the following premises: (a) it must facilitate nicely structured programs and data; (b) it must be readable; (c) it must actively assist in the detection and isolation of errors; (d) it must be compilable into efficient code; (e) it must allow the programmer precise control over the emission of instructions and allocation of storage; (f) it must be easily modifiable; (g) it must have an efficient compiler; and (h) it must be easily implementable. Although the resulting language is very much a product of the Project SUE environment, some of its features may be of more general interest.