As revealed in a recent judicial review challenge in relation to landfill tax repayments, HMRC has developed a new concept – ‘boundary pushing’ – for identifying conduct of which it does not approve but which falls short of avoidance. However, in a context where already established concepts such as avoidance and evasion are so slippery, it is time to abandon old favourites altogether and use words that more accurately distinguish between ‘acceptable’ and ‘unacceptable’ conduct. Anything short of this risks the continued haziness affecting – for the worse – the development of tax law and its practical implementation in the way intended by Parliament.