Vibrations in drillstrings have been a concern for many years. Equipment failures, especially those that occur in the bottom-hole assembly, can be linked directly to severe vibrations. Equipment, associated with directional drilling has become more and more complex and vulnerable to every mode of vibration. Reliability and maintainability have become important considerations in downhole tool design. This chapter considers the causes and control of mechanical vibrations along drillstrings, especially in the bottom hole assembly region.