Computing is unlike most technologies in that it has two systems of preservation: restoration, which it shares with every other technology, and simulation. Alas, restoration is not simple. Very few computer components can be made by hobbyists in their basement.Fortunately, there is the alternative: simulation. With the right virtualization software, old hardware can be brought back to life as programs running on PCs. The virtue of simulation is that old documentation is a lot easier to store than old hardware, and often the memories of techies reach far back. With the goal of mining some of that old documentation and memory, the Computer History Simulation Project is trying to create PC-compatible simulators for a wide range of significant computer hardware and software systems and publish them as freeware on the Internet. Examples of simulators currently available are the Data General Nova, Digital VAX, IBM System 3 and MITS Altair 8800.The problem these simulators address is that computing systems lose value quickly and are rapidly discarded, together with their applications and documentation. Ten years after a computer is brought to market, it might as well have never existed. This raises both practical and cultural issues. Often there will be some residual versions of those machines running somewhere in the world for which lack of support is a critical problem. Useful or significant data might be stranded on media that only those machines can read. Also, these machines represent important chapters in our history. It seems strange that we should know more about the technology of the 19th century than that of our own time, but older machines typically lasted longer and were better documented. For more information about the project, visit http:\/\/simh.trailing-edge.com.