Present day digital computers are too slow for the comprehensive real-time simulation of complex electronic systems such as those involving guided missiles. Computers appearing 2-3 years hence will be sufficiently fast for these applications. There is and will continue to be fertile application fields for analog-digital computer combinations. An existing large-scale (Univac Scientific 1103A-Epsco converter?Electronic Associates) analog-digital system is described. System design and specifications are discussed which relate to real-time speeds, necessary accuracies, digital computer programming, and synchronizing the digital computer with the remaining system components. Techniques for exploiting each computer's advantages are mentioned. Applications in the guided missile field and in other fields are discussed.