I find that a lot Leica shooters have this notion that flash should be avoided at all costs or take some pride in "natural light" photography... I personally think that's a bit narrow but hey whatever. So here's a thread for the few that don't shy away from flashes.... and may be looking for suggestions. My default have been Sunpaks 422D's (I have 4 of them, 1 with zoom head) with TR-Pak battery pack ( two of them) that have been modified to take RC batteries. The modified batteries provide very fast recycle times. Often with the Nikon SC-17 sync cord when used with the camera. The same set doubles with two basic umbrellas and stands with Cactus V triggers. On occasion, I need something really powerful with the umbrellas and I will bring along a Sunpak 622 pro (high trigger voltage!) using the same batteries. I've been using this set of 422Ds with all my cameras not just with Leica for a very long time. Cheap to find. Reasonable trigger voltage. Replaceable shoes without any tools. EXT-09 remote cord keeps the thryster over the camera. Auto and Manual to 1/16th. Available manual zoom head. Twist and tilt flash head. Runs on 4 AAs or TR-Paks. The 422Ds are missing the battery carriers that make it quick and easy to replace the AAs. They are smaller than many compact flashes of the same time period BUT they still feel large on a Leica M. My kit gets even more bulkier since I prefer to use them with the TR-paks.... which provide more than enough power and cycles. Next... the Leica SF24D. Small, compact, powerful enough, uses CR123A batteries (more on that later). It feels at home on a Leica M size and weight wise. At $350USD it doesn't bring a whole lot of value to the table... then again Leica's other flash is $600USD+. The SF-24D is TTL capable with the M's but I find it barely usable. The preflash is slow enough that it can cause subjects to blink when the exposure flash fires. In Auto mode, it works fairly well... controls can be slow on the back. Head doesn't not tilt nor swivel. Their choice of battery is a double edge sword. The CR123A batteries give more cycles and more power than the typical 2 or 4 AA's configuration of compact flashes AND they allow for a smaller unit. VERY NICE! On the other hand, make sure you are carrying an extra set... not as readily available as the AA batteries. There are some rechargeable CR123A batteries available via Ebay and impossible to find locally US. Mine work well enough.... I often use this flash off camera with the same SC-17 cord. Next... The Olympus FL-600R (just got it so still learning it). Close to perfect for the Leica M (oddly enough). Its smaller than most of the older style compact flashes since it was designed for the smaller micro 4/3 cameras. Its used in Auto or manual mode. Tilt and swivel with electronic zoom head w/ wide panel All of the custom functions and controls are easy on the back. It has the option to set the focal length readings to FF rather than the 2x crop so you don't get confused. Uses AA batteries yet still manages to be small. Powerful enough. Manual mode using GN with a recommended range readout. Even has a built in optical slave mode. For those with the new M 240, it even has a steady video light built right in as well. On the missing side of things.... It doesn't seem to have an option for a high voltage/capacity battery pack. Unfortunately, mine was dropped (yes.. on the second day of getting it used.. grr) and although I have repaired its shoe the unit no longer swivel's its head. So if you are a strobist, I say the 422D's are a good option. The well known Vivitar 285HV are also a good option (still available new) albeit the head doesn't swivel. If you are looking for a single flash with more options, the FL600R is surprisingly very well featured. I'm keeping an eye out for another FL-600R as I would like to have mine swivel.