If you are cycling after dark, legally you need front and rear lights. Even if you are cycling in the daytime, lights and bright, reflective clothing will make you more visible, especially in bad weather. As you'd expect, the more you spend on lights, the brighter they tend to be. Brightness is measured in lumens and varies from tiny back-up LED flashers, to off road lights with thousands of lumens, costing hundreds of pounds, which would rival a car headlight.
Lights and the law: Between the hours of sunset and sunrise, you must have at least one white front light and one red rear light, plus a reflector at each end of your bike and on your pedals.
Buying a light set usually works out cheaper than purchasing front and rear lights individually.
It's a great way to get a cheap set of back-up lights or a more substantial pair of main lights. What's more, you'll have a coordinated look to your bike if you opt for a set.
If you commute mostly on lit-up city streets, you will need lights to be seen by, rather than to see by.
Even so, the more lights you have, the more visible you will be to traffic so ideally you should have more than one light at each end, at different heights. For instance, attach a main light to your seatpost and have a back-up light on the rear of your helmet.
If you're a mountain biker and want to venture onto the trails after dark, you won't have any street lighting to guide you.
That's when super-powerful front lamps with rechargeable batteries come into their own. Lighter units have the battery built into the light body, while the longest lasting and most powerful lights have a separate battery unit which must be attached to your bike, in a bottle cage mount, for instance.
There are two main power sources for your lights: ordinary batteries which need replacing when they run out, or those with a rechargeable battery built in.
Rechargeable lights used to be far more expensive, but there are now many cost effective and compact units available which recharge from your computer with a USB cable. This makes them a brilliant option for commuters, who can boost them at their office desk during the day.
There are other ways to charge your lights. Dynamo lightings store charge from your rotating wheel. They are less popular these days and expensive to fit as you will need a new wheel built, but they require no other power source than your own legs. There are even a few solar power lights for totally carbon neutral cycling!
Finally, your lights are not the only way to make sure you can be seen at night. If you are a road rider, consider some high-vis clothing and accessories.
You don't need to deck yourself out head to foot in dayglo yellow as most cycling jackets and trousers will have some subtle reflectives built in. Alternatively, consider some high-vis accessories, like a rucksack cover or ankle bands.