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.