It depends what the device is, and what its voltage, current and frequency requirements are. These should be on a label somewhere on the device.
Electronics often have a wide enough tolerance that they will work without any transformer. If the current requirement is high then a suitable transformer will be big, heavy and expensive. If it has a mains-powered motor then the frequency will also be an issue as some types of motor designed for the US 60 Hz frequency will run at the wrong speed if connected to the UK 50 Hz supply, and transformers don't generally change the frequency.