The invention provides apparatus and systems for high availability execution of applications executing on one or more digital data processor platforms. For example, a system according to one aspect of the invention includes a first set digital data processors, each of which executes one or more applications. These are referred to as “primaries” or “primary applications.” A second set of digital data processors execute virtual machines, each of which, in turn, executes an image of a corresponding primary application. These are referred to as “imaged” or “imaged applications.” Each of the first and second sets may have one or more digital data processors. Agent functionality residing on and/or coupled to the digital data processors in the first and second sets maintains state/configuration information for each imaged application. That information, which reflects current or substantially current state of the corresponding primary application, is of such a nature as to permit the imaged application to take over applications processing, e.g., in the event the primary application fails or otherwise becomes unavailable. The agent functionality, according to related aspects of the invention, can monitor the primary applications (and/or the first set of digital data processors) and effect such a take over by a specific imaged application.