The phrase "plug-and-play" refers to hardware and software devices that, after being installed ("plugged in"), can immediately be used ("played with"). The system can be used for a specific application without requiring additional software programming or hardware design. The Essential Services Node (ESN) takes a big step in moving space systems to the "plug and play" environment of commercial systems. Systems that use the ESN to interface subsystems to the spacecraft bus (either MILSTD-1553B or MIL-STD-1773) provide a standard interface to both the sub-systems and the spacecraft bus. The ESN is a multi-chip module (MCM) that contains an ASIC, memory devices, and analog circuitry. The ASIC contains a microprocessor, a MIL-STD-1553B/-1773 bus interface, and many industry standard devices. The memory includes program-, data- and shared-RAM. The analog logic includes a 16-channel A/D converter, an 8-channel 1.0 mA current source, and an analog power strobing circuit. The ESN MCM is a low-power ( 100K), industry standard, spacecraft control and data system, which eliminates custom interfaces by choosing a set of industry standard interfaces. This paper describes how the ESN provides a standard interface to the spacecraft's bus and its sub-systems. It also describes the plug-and-play standard, Open Firmware, and our plans to port it to the spacecraft environment.