Mostly you should just carry a good tactical flashlight. By tactical I mean easy (for you) to quickly turn on and off one-handed, and possessing significant output. "Significant output" used to mean at least 100 lumens, but I prefer 500 lumens or more. I want my light to have a chance to temporarily blind an assailant, maybe even pee a little.

If you're already carrying a good light, I'd probably prioritize low-light training and practice next. If you're already doing that, you're way ahead of the curve. Tritium night sights work well for me.

I've got to agree with the advice from chaosmagnet. If you can't get some actual training/trigger time under low-light conditions there are some good videos (some really bad ones too) and that may give you some ideas. Find someone who actually knows how to use a light and handgun and work with them is the best bet.

I have trained with the 'harris' technique, the old fashioned FBI style and several others but you need to find what works best for you and your situation. I've got a light mount on the long gun but not on any handgun. However I always have a light with me. I got a purpose build extra mag holder that included a slot for my surefire flashlight and when asked about it I always maintain I would much rather have the light and one extra mag than two mags and no light.

EDIT: I forgot an important factor - from a cop's standpoint, a weapon mounted light equals pointing a weapon at what ever you want to illuminate - a light independent of the weapon lets you light up anything without also making you point a weapon at it too. Having both may be the better plan.

I got the original surefire back in the day (expensive but I was betting my life on it)and it was amazing - 100-160 lumens to light up the area or a 'bad guy' night or day - there are a lot of dark places inside even in the daytime. Right now my primary light kicks out 1800 lumens and I like it a lot. YMMV. Find something that works for you and practice with it.

Paul -

As pforeman says, a weaponlight should not ever be used to illuminate anyone or anything that you do not intend to shoot. A handheld flashlight is a necessity for low light operations. A weapon mounted light is great to have. When I’m carrying a firearm, I always have both.

Malkoff Devices ... many of you may already be familiar with the Malkoff modules and flashlights, but if not... they are not inexpensive, but if you have a quality LED light that you depend on....

the module is machined brass that acts as a heat sink to allow longer run time before your driver over heats, and the LED and supporting electronics is "potted", encased in resin... coupled with a well designed heat dissipating host flashlight body, heat is transferred for much longer use

https://malkoffdevices.com/collections/surefire-drop-ins-for-6p-g2-c2-etc-6-9-volts

MTC
As a former peace officer, in the mid 1970,s 80s . I had a stream light SL20 the most powerful flash light of the day, always carried it on my belt. For the above reasons.
However I am against any weapon light for the police,maybe exception for special teams. The local cops are not navy seals and most do not get enough firearms training as it is let alone waving a gun around with a light. YMMV

Malkoff makes great lights! I have maybe 8 or 9 complete Malkoff lights and more P60-type Malkoff dropins than I can readily count.

At least one large metropolitan police department banned weaponlights for patrol officers due to training issues. Around where I live a number of departments mandate weaponlights but also train with them extensively. I’ve never served in law enforcement, and I can focus my training time in a way that I suspect they largely cannot.

Chaosmagnet got it right - if you've got a light on your weapon, you better have a 'free' light to use as well. Back in the '70s - '80s I had an acquaintance who had done some extensive training with the RCMP on low-light shooting. His observation was weapons lights alone were a big problem and it was better to have a simple flashlight for the vast majority of tasks requiring a light. This was also in the day when using muzzle flash to see for night shooting was a technique...

An additional insight was that in most urban or built up areas there is usually quite a bit of ambient light - street lights, business lighting, parking lots, yard light etc. When you get into really rural areas like the boonies where RCMP may be and much like I was in out in the county there is often no light (think cloudy night, no moon and it's 2AM) You don't want to be looking for a lost kid with a light tied to your weapon and you sure don't want to be looking for a wounded animal without a light.

I still carry around a couple of flashlights daily even retired and when I was doing Corrections training the only other 'tool' used more than my flashlight were gloves... I always expected the rookies I worked with these last years to get and have a good light on them at all times.

To sum up: Get a good light to carry and a secondary one for a weapon (don't go cheap), carry it, practice with it, and get some practical training too.

Paul -

I picked up one like this from gouldusa.com - their part number 860. No affiliation, just a happy customer.

Streamlight SL20: I have mine still in the car. Big, bright, and good as an "impact weapon of opportunity."

This week I started experimenting with a belt-mounted flashlight carrier from L.A.G. Tactical (no affiliation). I’ve been carrying it behind my mag carrier on the support hand side, getting my flashlight out of my pocket with the intention to make it faster to deploy and perhaps a bit more comfortable. So far so good.