We have developed a program of complementary balloon-borne experiments to measure the Cosmic Microwave Background Radiation (CMBR) anisotropy on angular scales from 0.33 to 180 degrees between 2.3 and 23 icm (70 and 660 GHz or 0.4 and 4.4 mm).

Such measurements have become increasingly important for providing information on the initial conditions from which the large-scale structure of the Universe has evolved. The recent detections on large angular scales by COBE and our FIRS experiment have completed the discovery phase for CMBR anisotropy studies.

We now enter a detailed measurement phase which promises quantitative answers to some of the fundamental questions of structure evolution in our Universe: How did matter first distribute itself to eventually form the bubbles, voids and galaxy clusters that we observe? What is the amplitude of the quantum fluctuations which existed before the Universe entered the `inflationary' epoch, approximately 10^-35 seconds after the Big Bang? Is the bulk of the matter in the Universe composed of a new kind of non-baryonic, nonluminous matter, as proposed by Cold Dark Matter theories? These questions have guided us to formulate the following program of current and planned experiments.