The extensive application of mass spectrometry in organic chemistry began around 1960. There has been a great increase in the use of this physical method since that time. There are two main reasons for the burst of activity in this field. First, it is now generally appreciated by organic chemists that there are a number of instruments available which are able to volatilize the vast majority of organic compounds in which they are interested, to ionize the vapor, and hence to record the molecular weight by measurement of the mass to charge ratio. Second, it is also appreciated that the molecular ion, formed as described above, breaks down into charged fragments whose structures can be related to the structure of the intact molecule. It is the purpose of this article to illustrate briefly how the mass spectra of organic compounds can be obtained and presented and, most important, how they can be interpreted.