Like Gaul, a mass spectrometer may be divided into three parts: source, analyser and detector system. The status of each is reviewed. Recent developments concerning the ion source region include improved techniques for chemical ionization (c.i.), field desorption (f.d.) and negative ion formation. There has also been significant work with pyrolysis probes, high pressure liquid chromatography (h.p.l.c.) interfaces and improved fast pumping for use with atmospheric pressure ionization (a.p.i.) and direct coupling of capillary column gas chromatographs. Commonly used analysers are now either the quadrupole r.f. filter or single/double focusing magnetic deflexion instruments. Magnetic instrument developments reviewed include improvements to sensitivity (Z-focusing and image curvature correction), higher scan speeds (laminated magnets), mass indication and marking, peak jumping when using multiple ion detection (m.i.d.) and reverse geometries (permitting mass analysed ion kinetic energy spectroscopy; Mikes) and linked scan (providing metastable ion information). Detection systems are discussed in detail and include consideration of computer data acquisition and display. Important are simultaneous foreground/background systems, library search, wide dynamic range, precision mass measurement at low resolution (higher sensitivity) and multiple ion detection at either low or high resolution. The use of linked scan and Mikes as specific ion detectors is considered.