The latest version of Gens, found at http://gens.consolemul.com/ is currently 2.11 for the Official Gens, © 1999-2002 Stéphane Dallongeville, so download, and use Gens. 2.11 before seeking help, as many games were fixed in his latest version
Gens 2.11 requires DirectX 7.0 or later, which can be found at Microsoft's Web Site, DirectX 8.0a being the last official DirectX version for Windows 95
Gens emulates the Sega Genesis, CD, and 32x systems. It doesn't handle Sega Master System, Game Gear, Saturn, or Dreamcast games.
the Minumum requirements for Genesis emulation are:
for Sega CD emulation, up these to:
for 32x emulation, 800 Mhz CPU+the bios files

We highly recommend you have at least a 300 Mhz Pentium processor with a good Video card (TNT, 3dfx, Nvidia, ATI) for Genesis emulation, and a 333 Mhz Pentium 2 with a Quad (4x) speed CD-ROM drive for Sega CD emulation, and a 1Ghz processor for 32x emulation

Help! My {insert game name} doesn't work!
First, make sure that the rom in question, actually opens up in WinZIP. if it doesn't, there isn't much you can do to fix it, except delete it and try again. also, Gens only supports ZIP compression. so, rom.ace, rom.7z, rom.rar aren't going to work. the space savings for them, vs. WinZIP is too little for Stef to care, so don't waste his, or our time with such requests. You can always re-compress them into WinZIP format, after decompressing them, and they should work. Another thing, some games weren't properly dumped (copied from cart to computer), and Gens, faithfully does what a real Genesis would do, and lock up (at worst) or loop through a music track with a blank screen (e.g. Shining in the Darkness bad dumps), which can usually be sorted out with GoodGen. If you dislike command prompts, use GoodWindows for good dumps that work, or don't work, we have 2 lists available, a 32x and Sega CD compatibility list and a 32x compatibility list I must stress, though, that 32xCD games, like Night Trap 32x, which use the 32x for enhanced Full Motion Video sequences (32,768 colors instead of 512 tops for Sega CD), DON'T work at all. and Stef has little interest in adding it, as his time is focused elseware. Virtua Racing, and other cartridges with Special chips, are not yet emulated, as there is very little information about the chips.

My CD-Rom Drive isn't selectable in Options -> Current CD-Rom Drive, not even listed!
If Gens cannot see your CD-ROM drive. it is because your CD-ROM drive's ASPI layer is not properly installed. Linux users, read the last paragraph. you can get the ASPI layer, for your appropriate operating system, from your CD-ROM drive's manfacturer's web site, from Adaptec/Roxio Easy CD Creator, or if you have to, install ForceASPI as most have good success with it. older ASPI layers seem to work better, but try the newest ones, and work your way backwards to a working version
ForceASPI can be downloaded here (note - direct link to file download)
if this still doesn't work, install Easy CD Creator's ASPI Layer, then Install ForceASPI ASPI layer over it. yes, over, replace the Easy CD Creator one. seems like Easy Cd Creator does most of the installation, but leaves a few loose strings, that Force ASPI handles. and if it still doesn't work? reinstall Easy CD Creator. and if still nothing, post, and include system specifications (like CD-ROM drive make/model, O/S, Gens Version, and include what you did too so we don't refer you back here) and if it still doesn't list your C drive, or Virtual Cd Drive (e.g. Daemon Tools mounting bin/Cue image), try Downloading ASPI Layer Fixer http://www.dcemulation.com/neededtools.htm it's on a Dreamcast site, how nice of them!
Linux users, running Gens via WINE, creditted to Tsunami: lately.. wine has gotten really accurate and has a lot more speed and direct x compatibility, but some of you (I bet I'm the only linux user here) may notice that there arent any drives detected... simple fix to this.... do a su root command then insmod sg to make this change a permanent one.. edit your /etc/modules file and add sg in... so it'll always boot.. simple fix.

I get a message about the RAM cart RAM Cart not Initialized
the RAM cart is an extra cartridge, that allows more space to save stuff. Lunar uses it, and one of the Lunar games needs it, to access the last part of the game (so much memory taken up by the game itself, leaving none for saves. this is where the RAM cart comes in.
The Sega-CD/Mega-CD came with 8kb of internal BRam for save games, this only allowed fairly basic save game features and is often a problem on real hardware as games begin to argue over the limited space. External SRam carts were also available, ranging from 8kb to 64kb in size. The small amount of internal BRam is less of a problem on Gens because it creates a new virtual machine, and separate internal BRam file, for each game. However to offer greater flexibility, and more save slots for each game, Gens also supports "virtual ram carts". Each BRam file created by Gens stores both the internal BRam and external SRam data in the same file.
You can set the size of the virtual ram cart to None, 8kb, 16kb, 32kb or 64kb through the options menu. It is recommended that you always set this to 64kb however because if you later wish to increase the size of the virtual ram cart you will need to reformat it and any existing save data will be lost.
To setup the virtual ram cart for your Sega-CD/Mega-CD game boot the CD or load your CD image as normal. When the Sega-CD/Mega-CD logo screen appears press the A, B or C button to access the advanced menu and then select the memory management features by moving the pointer to "option" or "memory", depending on the bios version being used. Follow the on screen instructions to format your virtual ram cart then start your game as normal.
Don't forget that you will need to set up the virtual ram cart function separately for each Sega-CD/Mega-CD game that you wish to use it with.

How to play Sega CD games?
If you are playing from a real Sega CD, than you should have little or no problems. ISO CD images of Sega CD games, need ALL the audio tracks, in MP3 format. specifically, the MP3 files have to be CBR (Constant Bitrate) format. VBR (Variable Bitrate) MP3 files are unpredictable with Gens. the size difference is not much between the 2, and many players have some problems with VBR anyway, so CBR is safer. the Audio tracks, have to be named specially. 
For Gens to correctly identify any .mp3 audio tracks for your CD images they
have to be placed in the same directory as the CD image file with the same
file name as the CD image followed by a track number. For example if you are
using a CD image called Blaster.iso with three .mp3 files you would need to
put all the files in the same directory and rename them like this:
  
     Blaster.iso
     Blaster 02.mp3
     Blaster 03.mp3
     Blaster 04.mp3

There are a couple of important points to remember when setting up your CD
images & .mp3 files for use with Gens:

     * The CD Image should NOT be numbered, it won't work if you have
       "Blaster 01.iso" instead of "Blaster.iso"!
     * The CD image acts as track one so the first audio track will always
       be track two+.
     * The track numbers must also be a TWO DIGIT number: 02, 03, ..10 etc,
       NOT 2, 3,...10.
     * The space between the file name and the track number is optional.

[+ Gens actually calculates the track order by looking at the numbers at the
   end of the .mp3 files. You can use any numbers you want, for example 10,
   27, 32... however it's easier to rename them in the way shown above.]

after you have the game ISO and MP3 files, in the same folder, and named accordingly, you also need the BIOS files. they can be found here then, tell Gens where they are, in Options -> BIOS files the Sega Genesis/Megadrive BIOS file, is not required. Its only usage, is a special intro in Gens. Reading the Gens.txt will shed some light on this.

32x games don't work!
you need the 32x BIOS files, which are here, and then in Gens, Options -> BIOS/Misc. Files

32x is so Slow and Choppy!
of course it is :) , the 32x is one complex computer, featuring 2 Hitachi SH-2 Processors @ 22 Mhz, AND the Genesis system also has to be emulated (7.68 Mhz 16-bit Motorolla 68000 and 4 Mhz Z80 chip, and the video and audio circuitry, too) 32x emulation is sufficient on a 800 Mhz CPU, but is much better on a 1Ghz+ computer.