Deep spaces mission of the future will require propulsion systems with specific impulses far greater than those currently achievable. The highest specific impulse of chemical rockets is a little over 450s and that of operational ion engines vary between 3000-5000s. However, for missions to the outer solar system and beyond, specific impulse of over 10,000s is desirable. This book explores the different technologies present, and in development to achieve this goal. The analysis involves use of computer programs made on FORTRAN language, to calculate and compare the various parameters defining these technologies.