The explanation of the formal duality of Kerdock and Preparata codes - one of the outstanding recent results in applied algebra - is related to the discovery of large sets of quadriphase <em>sequences</em> over Z₄ whose <em>correlation properties</em> are better than those of the best binary sequences. Moreover, the correlation properties of sequences are closely related to <em>difference</em> properties of certain sets in (cyclic) groups. <br/> Most of the articles collected here contain descriptions of the <em>connection</em> between difference sets, sequences and correlation properties of sequences. There are two more elementary introductory articles: an introduction to difference sets (by two of the editors), and an introduction to the correlation of sequences (by Solomon Golomb).