For C—N and C—S bonds there are now enough experimental results to enable bond-order/ length relationships to be proposed which can be used to correlate theoretical calculations of the distribution of π-electrons with experimental measurements of interatomic distances with sufficient accuracy to give critical guidance in further theoretical investigations. For these bonds, as for C—C bonds, consideration of π-bonding only accounts for much of the observational data relating to interatomic links intermediate in length between single and double bonds; the effects of hyperconjugation, polarity, hybridization and other environmental factors seem to be so small that they cannot be studied satisfactorily until an experimental accuracy of 0.01Å or better can be obtained consistently. These remarks cannot be applied with the same certainty at present to C—O bonds where there is some evidence that environmental factors are more important. An interesting feature of recent work has been the accumulation of evidence that trigonal hybridization of nitrogen atoms is much more common than might have been supposed a few years ago.