This article reviews studies dealing with the content of electronic (e-) cigarettes. Based on measurements of the e-juice, the inhaled and the exhaled vapour, it is sound to assume that smoking e-cigarettes might have much less detrimental health effects than smoking conventional cigarettes. However, propylene glycol and glycerine are abundant in e-cigarettes and although they are generally perceived as relatively harmless, the long-term effects of heavy exposure to these substances are unknown.