This book enables readers to design lens systems having useful characteristics. The text covers the basic theory of the motion of charged particles in electrostatic fields and describes several methods for the calculation of the potential and field distribution for various electrode geometries. Two methods, the Bessel function expansion method which was developed by the author and his students and the nine-point implementation of the finite difference method, are given special emphasis because they are particularly appropriate for implementation by the inexperienced user. Other methods are discussed in less detail and reference is made to websites from which demonstration programs for these methods may be obtained.