[not before noun] giving attention or thought to what you are doing so that you avoid hurting yourself, damaging something or doing something wrong Be careful! careful to do something He was careful to keep out of sight. careful not to do something Be careful not to wake the baby. careful when/what/how, etc. You must be careful when handling chemicals. Be careful what you say. careful of/about/with something Be careful of the traffic. Please be careful with my glasses (= Don't break them). I’m very careful about washing my hands before eating (= I make sure I do it). careful (that)… Be careful you don't bump your head.