The ISO mission properties described in Part I set the overall constraints to the observing programmes. The astronomical conventions and limitations are discussed in Part II. This Part III deals with the actual observations within the framework defined in the preceding two parts. The scope of this part is limited to the ISO satellite as a whole. Only the modes available by the spacecraft and overheads due to the spacecraft operations are presented. The instrument part of a specific observation is described in the OBSERVER'S MANUAL or their Addenda of the corresponding instrument. Those documents also give details on how to calculate observing times for Phase 1 proposal submission, including worked examples. For CAM, LWS and SWS instrument time estimator software should be used, where for CAM and SWS there is the caveat that the times are really only estimates. For PHT observing times can be calculated without a time estimator by following the recipes explained in the Addendum to the ISOPHOT OBSERVER'S MANUAL.