The execution of observations in Service Mode is an option at the European Southern Observatory Very Large Telescope. In this operations mode, observations are not scheduled for specific nights, they are scheduled flexibly. Each night observations are selected from a pool of possible observations based on Observing Programme Committee (OPC) priority and the current observing conditions. Ideally, the pool of possible observations contains a range of observations that exactly match the real range of conditions and the real number of available hours, so that all observations are completed in a timely manner. Since this ideal case never occurs, constructing the pool of observations must be done carefully, with the goals of maximizing scientific return and operational efficiency. In this paper, basic ESO Service Mode scheduling concepts are presented. A specific VLT focus is maintained for most of this article, but the general principles are true for all ESO facilities executing Service Mode runs.