Introduction This EEMUA Publication is intended as a guide for non-analyser specialists to assist in the development and implementation of on-line analyser applications. It covers how to determine if an on-line process analyser is justified and provides details of the information required to ensure the analyser is correctly selected@ purchased@ installed and maintained. The intended users and uses of this Publication are: Companies with little or no experience in applying on-line analysers; Companies who do not have experienced on-line analyser specialists; Instrument engineers with limited experience with on-line analyser systems; Vendors@ to better understand how end users apply analysers; To help ensure analyser systems integrators are correctly specifying on-line analysers; and To help ensure analysers in process package systems are correctly specified and installed. This Publication is not intended to cover: Detailed analyser system design and installation guidelines - refer to EEMUA Publication 138 Design and installation of on-line analyser systems(1); Detailed procurement and bid evaluation guidelines - refer to EEMUA Publication 226 Design and installation of on-line analyser systems: Guide to technical enquiry and bid evaluation(2); Guidelines for analyser maintenance - refer to EEMUA Publication 187 Analyser Systems: Guide to maintenance management(3); or Guidelines for analyser calibration and validation ?C refer to EEMUA Publication 175 EEMUA Code of Practice for the calibration and validation of process analysers(4).