Antiprotons have been proposed as potential modality for particle beam cancer therapy by Gray and Kalogeropoulos in 1985. This proposal was based on the enhancement of physical dose deposition near the end of range due to the annihilation of antiprotons when captured by a nucleus and the expectation of an enhanced RBE of this additional dose. Starting in 2003 the AD-4 collaboration at CERN has studied biological effects of antiproton beams on V79 Chinese Hamster cells and human FaDu cells. The AD-4 collaboration has developed relative and absolute dosimetry for pulsed antiproton beams. Data from these measurements were used to benchmark the FLUKA Monte Carlo code, which then has been used for calculations of physical dose inside and outside of the primary antiproton beam. From clonogenic survival studies on the different cell lines mentioned above we have determined biological effective dose ratios (BEDR) for peak to plateau of antiproton and proton beams of identical energies and beam conditions. Using a clinically relevant energy of 126 MeV we generated a 1 cm deep spread-out bragg peak (SOBP) and measured survival vs. depth for both cell lines. Together with the capability to determine the absolute dose deposited at each depth point we are now in the position to extract relative biological efficencies (RBE) vs. depth. Using Monte Carlo calculations and various biological assays we also have studied the effect of the peripheral dose deposited as a result of the isotropic annihilation field emanating from the Bragg peak. We will describe the current status of the experimental program and will speculate on future developments.