Service observing has several global goals:

• Maximize science efficiency by executing the programmes with highest scientific priority first and under the required observing conditions;
• Maximize operational efficiency by sharing calibration data between programmes, and by helping infrequent users of complex facilities in optimizing the use of the allocated observing time;
• Maximize the scientific use of telescope time by having appropriate programmes ready for execution under a broad range of observing conditions;
• Maximize the scientific productivity of the facility by means of the reuse of the data, made possible by building uniform data sets accessible through an archive.

The key concept in optimizing the use of the available time to produce the highest scientific return is Flexible Scheduling, i.e., the capability to continuously adapt the observing schedule to the external, often unpredictable conditions. For this reason, and unlike in the case of space observatories, it is not generally possible to foresee the precise date when a given observation or programme will be carried out.

An obvious limitation of Service Mode observing is the impossibility for users to take decisions on the execution of their programmes in real time, for example based on a specialized scientific assessment of the outcome of a new observation. Programmes in which real-time decisions are needed for their scientific success are thus deemed unsuitable for Service Mode observing, and are normally carried out in Visitor Mode instead. Runs with a justified need for Visitor Mode observations and whose duration is smaller than one night may be scheduled in the co-called Designated Visitor Mode, depending on the technical feasibility reports. Designated Visitor Mode observations are scheduled on specific dates/slots as if they were regular Visitor Mode runs, but they are executed by an ESO staff member, in close contact (e.g. via phone, Skype or video link) with the Principal Investigator, or someone the PI designates to serve as the liaison with the Observatory.

In order to meet the goals stated above while also serving the needs of a diverse astronomical community, it is necessary to have a structured system in place that guides and monitors programmes throughout their entire life cycle, from the initial submission of the observation programmes to the use of their data for archival research in the future. The ESO Data Management and Operations Division (DMO) has been given the responsibility to develop the VLT Data Flow System (DFS) to supply the tools and services necessary to achieve these goals.

Managing Service Mode programmes through their life cycle is the joint responsibility of:

• The Observing Programmes Office(OPO),
• The User Support Department (USD),
• The Paranal Science Operations Team or the La Silla Science Operations Team,
• The Back-End Operations Department (BOD) and its Quality Control (QC) Group, and
• The Science Archive Facility (SAF).