Over the last few years, there has been a remarkable convergence among the fields of ultrafast optics, optical frequency metrology, and precision laser spectroscopy. This convergence has enabled unprecedented advances in control of the electric field of the pulses produced by femtosecond mode-locked lasers. The resulting spectrum consists of a comb of sharp spectral lines with well-defined frequencies. These new techniques and capabilities are generally known as "femtosecond comb technology." They have had dramatic impact on the diverse fields of precision measurement and extreme nonlinear optical physics. This book provides an introductory description of mode-locked lasers, the connection between time and frequency descriptions of their output and the physical origins of the electric field dynamics, together with an overview of applications of femtosecond comb technology. Individual chapters go into more detail on mode-locked laser development, spectral broadening in microstructure fiber, optical parametric amplification, optical frequency metrology, optical atomic clocks, ultrasensitive sensors, carrier-envelope phase dynamics, high field ionization of atoms and generation of attosecond high-harmonic radiation. To provide readers with the most recent, direct, and comprehensive information, the chapters are written by the international researchers who have led the development of this field. This book provides an introduction to those new to the field and is at the same time a resource for experts.