In this book, we have attempted to produce a reference on high resolution focused ion beams (FIBs) that will be useful for both the user and the designer of FIB instrumentation. We have included a mix of theory and applications that seemed most useful to us. The field of FIBs has advanced rapidly since the application of the first field emission ion sources in the early 1970s. The development of the liquid metal ion source (LMIS) in the late 1960s and early 1970s and its application for FIBs in the late 1970s have resulted in a powerful tool for research and for industry. There have been hundreds of papers written on many aspects of LMIS and FIBs, and a useful and informative book on these subjects was published in 1991 by Phil Prewett and Grame Mair. Because there have been so many new applications and uses found for FIBs in the last ten years we felt that it was time for another book on the subject.