Given the extremely high bunch crossing rate foreseen at the Large Hadron Collider (LHC) and the general-purpose nature of the ATLAS particle physics experiment, after the hardware-based first level trigger, an efficient and flexible trigger software is needed for the online selection of events. This filtering of events is organized in two levels: the second level trigger and the event filter. Both levels are referred together as High Level Trigger (HLT). A coherent approach to event selection across the HLT has been taken. Thus a common core software framework has been designed to maximise the usage of offline interfaces and software components, whilst allowing sufficient flexibility to meet the different interfaces and requirements of the two different levels, notably those of performance and robustness. This paper describes the architecture and high level design of the selection software and shows how the implementation meets the challenges of the ATLAS environment.