A modular MEMS packaging and interface concept for custom-designed MEMS in mid-scale volumes was developed. Modular units include functional components, e.g. sensors, microactuators or signal processing components. Various systems can be build up by combinations of these modular units. A novel packaging design, the Top-Bottom Ball Grid Array TB-BGA, allows the 3D packaging. Standardized interfaces are necessary to combine modules from different suppliers. Geometric, electric, fluidic, optic and communication interface have been developed. Several realization technologies are discussed and an example for application is given.