This book gathers together chemical ideas which are important for understanding how drugs act and how new drugs may be developed. Students meet some of these ideas in courses in chemistry, biochemistry and pharmacology but in my experience they find it difficult to put together information often acquired in different years and from different departments; they need it set out in a book. I believe also that it helps if they can see how these ideas may be applied to numerical problems so I have included examples (with answers), many of which are taken from research work. These should therefore provide a chance for the student both to test his or her own understanding and to see the sort of results which form the bricks of which any experimental science is made. In all the problems the arithmetic takes less than ten minutes with a small calculator which has exponential functions and logarithms.