Theoretical aspects of singly polarized hadron-hadron collisions

The special role played by singly polarized high-energy hadron-hadron collisions in Spin Physics is discussed: In such processes, the measured and the calculated quantities can be and have been directly compared with each other --- without data-extrapolation and without sum rules. It is in this kind of processes, where significant asymmetries (up to 30-40\%) have been observed. It is also in this kind of processes, where the obtained data and the predictions of the conventional theories dramatically disagree with each other. Attempts to understand the existing data are briefly summarized. Predictions for further experiments are presented.