CAMS-84 is a sub-project of CAMS, dealing with the validation of the service products. CAMS-84 provides 3-monthly updates of validation reports for the global and regional services. The validation is based on a large number of observations and measurement techniques, including surface in-situ, surface remote sensing, observations by airplanes, balloon sounding, observations from ships and satellite observations. The three-monthly cycle of the validation reports adds constraints on the timely availability of the observations, with a deadline roughly one month after sensing. This document serves as a reference for the validation reports, in order to provide the traceability for the independent observations used in the validation work. The two main aspects discussed are: 1. A description of the observations used, including the list of contributing stations, observation networks, measurement techniques, QA procedures, and error estimates. 2. A description of the methods to compare these observations with the CAMS modelling and assimilation products. The focus of this document is on the evaluation of the real-time global service for reactive trace gases, aerosols, and greenhouse gases. Observations used for the reanalysis will be distinguished from observations used for the quarterly reports but are not discussed in this version 4 of the document.