I tried to keep the title post nice and short in the hope that this thread is as long lived as the coding one. So here's a slightly longer one with more information (and also more of my opinions you've heard three times before)

The mdl format was one of the first 3D model formats for games, and naturally is a little less advanced than anything seen today. Here's a rundown of the most important mdl limits:

1000 vertices
2048 triangles
256 frames

The other important limitation of the mdl format is that the vertices are stored with a precision of 8 bits per coordinate axis. The vertices are in effect "snapped" to a grid of 256x256x256 points. This means you can't create intricate detail in your model without it being lost when it moves. It is worth noting that the origin and spacing of this grid can be specified, so the level of detail which can be retained is relative to the overall size of the model.

Editing

One of the greatest barriers to making models for Quake 1 is the lack of good tools. The best tool which works with .mdl format natively at the moment is QMe. You can grab the full 3.0 from:
http://www.fpsbanana.com/tools/2544
and you can upgrade it to version 3.1 with the patch in
http://www.xs4all.nl/~renep/quakeme/qme31_p2.zip

Although QMe is useful for this reason, I'd personally recommend working with the mdl as little as possible. It is generally easier(in my opinion) to create your model in another format entirely, preferably one with a powerful editor which saves the vertices at high precision. Then just export the model to mdl each time you want to test it, much like you compile a new copy of the BSP each time you add to a map.

QuArK ( http://quark.planetquake.gamespy.com/ ) also handles .mdl format natively, as well as other more popular formats such as MD3. I've never been able to use it for any serious editing, but found it a useful tool for converting file formats. So it sits somewhere between working directly with the .mdl and just a waystation for converting files.

If you work in Maya, you are fortunate to have the most direct route to export to .mdl. Lunaren has written:http://lunaran.com/files/lunmodelgen.zip
One package which handles everything directly!

If you work in Blender, you can give a script for exporting to mdl directly a go:
http://www.btinternet.com/~chapterhonour/mdl_export.py
I wrote it a while ago, for blender 2.4.4, and I don't intend to ever revisit it. But it's written in python, so it should be straightforward for people to modify themselves. Alternatively, you can export your model to MD3, using either http://tremx.svn.sourceforge.net/viewvc/tremx/trunk/blender/
or http://cube.wikispaces.com/MD3+Export+From+Blender+Tutorial

Once you have an MD3 of your model, you can run it through QuArK to convert it to mdl. Alternatively (blowing my own trumpet here) you can use "MD3 to MDL"
http://www.btinternet.com/~chapterhonour/md3tomdl.zip
This is my new work in progress, and I welcome any feedback on it. It's a simple command line tool, requiring only a short text file and an MD3 to work from. I'm planning on extending the control file format to include things like model flags, multiple skins, named frame ranges and stuff, so suggestions of what could be useful will not go ignored.

Since MD3 is so well supported amongst editors, you can use the above to get to .mdl format from most major 3d modelling packages. If you don't have any of the more expensive programs (like me), then you could do a lot worse than getting a copy of Gmax. It's a stripped down version of 3d studio max intended for making gaming models, and it's free. Official support for the idea ended a few years ago, and http://www.turbosquid.com/gmax took over distribution. Make sure you also download the Tempest Game Pack, to get the md3 export you will need. Personally, I can't do without gmax...


This thread is pretty much a continuation of http://www.celephais.net/board/view_thread.php?id=60262 , a thread I'd forgotten about until I started researching for this post. Hopefully putting all this information at the top of the thread justifies starting a new one.

Finally, if the low polygon requirements of Q1 seem constraining, take a look at:
http://boards.polycount.net/showthread.php?t=41232
The things some of those posters do with 500 polys are just amazing.