They've been around seven years and 1.3 million people use them in Britain alone.
Yet only now are health officials answering the key question: How should e-cigarettes be regulated? And how safe and effective are they?
The Medicines and Healthcare Regulatory Authority says nicotine levels can vary from what is labelled and from batch to batch, casting doubt on how useful they are to those aiming to quit.
Their answer is to say the manufacturers should have to apply for a medicines license, which is a way of checking their safety, quality and effectiveness.
Until now e-cigarettes have been unregulated. They can be advertised and sold anywhere - including to children.
Pharmacists say sharp marketing practices have mimicked "the bad old days of glamorising smoking".
Licensing will mean e-cigarettes can only be advertised for cutting down or quitting - not as a ‘lifestyle choice’.
The BMA has welcomed the decision, saying people smoke for nicotine - but die from tar. E-cigarettes seemed to be the answer.