I have discussed punctuation with many hundreds of scientists. Time and again, I have been told that no-one has explained to them the rationale and system behind punctuation in English. That is a severe condemnation of the English-teaching profession. In partial redress of the situation, here is a brief rationale for careful punctuation in scientific writing. A full account of the system would fill almost the whole of this book, so I have mentioned in this chapter just one or two points that are vital to clear writing and easy reading. For detailed guidance on punctuation for scientific writing, see my book Full Marks.11 For discussions of punctuation in general contexts, I recommend the short sections in Modern English Usage12 and The Complete Plain Words,13 or the short books Mind The Stop14 by G.V.Carey and You Have a Point There15 by Eric Partridge.