Bike lights are a must as soon as the sun goes down, so that you can see the road - and so that others can see you. What you need depends on what you do. Mountain bikers need lights that pack a punch to light up the trails in front, while road bikers and commuters will need powerful front lights. At the back, a minimum of one rear light should be used. Brighter is better (unless it dazzles oncoming drivers), and flashing modes help keep you safe by attracting attention.