The cover picture of this volume displays one of the most advanced products of he- lioseismic research: a view into the deep interior of the sun, revealing its distinctly non-uniform rotation throughout the entire depth of the convection zone. Just over 20 years ago, the first successful helioseismic experiment disclosed an increase of ro- tation velocity in the uppermost one dozen megameters below the photosphere. The stunning progress in depth and detail highlighted by the cover diagram (and by others shown in this volume as well) was made possible by considerable advances in instru-...