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Microelectronic ion sensors based on monolithic silicon integrated circuit (IC) and hybrid circuit technologies have been the subject of considerable research and development over the past 15 years. This paper reviews the conceptual background and history of both kinds of device, comparing their operation with those of conventional ion-selective electrodes and coated-wire electrodes. Attention is focused on the interfacial processes involved in the ion-sensing mechanism of microelectronic devices, with particular reference to the significance of models based on either ideally blocking or nonblocking mechanisms. This is a matter which has important theoretical and practical consequences for both silicon IC-based microelectronic sensors such as the ionsensitive field-effect transistor (ISFET) and hybrid circuit based devices. Practical problems associated with effective encapsulation and the provision of a suitable reference electrode are also considered.