NIRSPEC is a recently funded, high-resolution, 1 - 5 micrometers cryogenic spectrograph for the Keck II telescope. The design of this new instrument is based on 1024 X 1024 InSb arrays and provides resolving powers of R equals 2,000 in non-cross-dispersed mode and R equals 25,000 in echelle mode with typically 5 to 6 orders on the array covering 60 - 90% of the selected waveband, J, H, K, or L, in a single observation. Later, even higher resolution can be achieved by using the proposed adaptive optics facility at Keck II and replacing some of the internal modules of NIRSPEC. This paper gives a brief description of the proposed design concepts, and a discussion of the detector and system constraints required to achieve the scientific goals of the instrument.