A femtosecond laser is a laser which emits optical pulses with a duration well below 1 ps (ultrashort pulses) ie: in the domain of femtoseconds. It thus also belongs to the category of ultrafast lasers or ultrashort pulse lasers. The generation of such short pulses is nearly always achieved with the technique of passive mode locking. The growth of the femtosecond duration range is certainly one of the most brilliant achievements of laser physics. With progress in ultrashort ultraintense laser technologies the peak power of a laser pulse increases year by year. These new instruments accessible to a large community of researchers revolutionised experiments in nonlinear optics because when laser pulse intensity exceeds or even approaches intra-atomic field strength the new physical picture of light-matter interaction appears. Laser radiation is efficiently transformed into fluxes of charged or neutral particles and the very wide band of electromagnetic emission (from THz up to x-rays) is observed. Ultrashort laser pulses, with durations approaching the timescales of fundamental atomic and molecular processes, are useful in a range of scientific disciplines. They deliver energy so quickly that new processes and observations are possible. Their short duration allows them to probe delicate living structures without destroying them and make material modifications on the micron scale with minimal or precisely controllable heat effects. This field of researches is in rapid progress now. Femtosecond-Scale Optics provides depicts of current developments and original results in some specific areas of this very wide scientific field. This text will be of interest for those specialized in the subject of laser-matter interactions.