Spectral observations of ions of beryllium in the JET plasma are described and are interpreted in terms of the interplay of atomic reactions and local plasma conditions. The methods of generalized- collisional-radiative theory are extended and used to provide a quantitative model. Emphasis is placed on the role of metastable ion states. The diagnostic scope of beryllium observations is explored for the influx ions Be+0 and Be+l in the visible spectral range, for Be+2 in the XUV, and for Be+3 by charge exchange spectroscopy. Useful results for general diagnostic interpretation are presented in tables and graphs. The calculation procedures and data form part of the 'Atomic Data and Analysis Structure' in use at JET.