Smoking is the main cause of death in the world today. In response to this negative trend, alternative devices to regular cigarettes - electronic cigarettes (e-cigarettes), are being produced. The basic parts of every e-cigarette are: battery, atomizer and filling, and they work on the principle of heating of e-liquid which contains nicotine and imitates the conventional cigarettes. E-liquid consists of nicotine, glycerol, propylene glycol and flavor enhancers. Researches have shown that these compounds are harmful to the human body and negatively affect the cardiovascular system, oxidative stress and inflammation, and the respiratory system. In addition to the mentioned compounds, some compounds present in the tobacco smoke of conventional cigarettes were also detected in the e-cigarette aerosol: tar, formaldehyde, acrolein, acetaldehyde and nitrosamines, some of which have been identified as carcinogenic compounds. This final paper describes the working principle of e-cigarettes, their development over the years, the chemical composition of e-liquid and compounds present in the liquid, the toxic effects of e-cigarettes on the human body, and a comparison of the toxicity of conventional cigarettes and e-cigarettes.