Abstract: The detection and identification of metal items is effected by a marginal oscillator the resonant circuit of which has an inductor surrounding a path along which any metal items are caused to travel. The frequency and amplitude changes in the oscillation produced by the oscillator due to the presence of a metal item in the inductor are compared with the changes caused by items of known metals and the identity of the metal of the item is derived from the comparison. Continuous or periodic drift correction of the frequency and amplidude values may be effected using a reference coil. Among other examples the invention may be used to monitor metallic particles in a lubricant or to identify larger items such as coins.