OBD, or On Board Diagnostics, is a system that monitors the car's sensors to detect when there may be a possible malfunction. It's good if you have a stock engine and tells you when your engine isn't running right. That said, when you start modifyng engine components with the aim of making more power, the OBD system may not take kindly to some things, and will light up that nice check engine light that we all love so much. OBD II is a more stringent version of OBD, and monitors more sensors and has more control over the engine than OBD-I does. The performance-minded induvidual prefers OBD-I or non-OBD because they leave more options for tuning. If your car is OBD-II, and you want to run OBD-I, you can get a conversion harness that will let you run an OBD-I ECM, and you won't have the additional constraints of an OBD-II ECM. If your car is OBD-II and you have an OBD-I engine, you need to convert to an OBD-I ECM. Or, you can convert the engine to OBD-II(emissions equipment, etc) and run an OBD-II ECM.