The principles of time-of-flight mass spectrometry (TOFMS) are described with a view to understanding the strengths and weaknesses of this method of mass analysis in the context of current applications of mass spectrometry and the more familiar scanning instruments. Fundamental and instrumental factors;affecting resolving power, sensitivity and speed are examined. Methods of gating ion populations and the special requirements in the detection and digitisation of the signals in the TOFMS experiment are discussed.