If you worry all the time about getting your bike light stolen (because they're so easy to steal) consider the KiBiSi iFlash One bike light. It's made of two parts held together by a strong magnet; one part attaches to the bike via seat post, stems, or handlebars; and the other is the light itself. Stick the light to the attachment and it lits up. You can also attach the front and back lights together for easy storage. It's beautifully designed and won the "reddot design award best of the best 2013".