This text focuses on the advances in our understanding of solar convection and activity, and on new methods and results of helioseismic diagnostics of the internal structures and dynamics of solar convection and active regions. The methods of local helioseismology (time-distance tomography, ring-diagram analysis, acoustic imaging, and holography) have provided promising results on the deep structure of large-scale convection and flows, emerging active regions, and sunspots. The book includes reviews and contributed papers presented at the SOHO-9 Workshop held on July 12-15, 1999, at Stanford University, California, USA. It provides a comprehensive overview of the current status of the field and presents new ideas and approaches to helioseismic diagnostics and theoretical modelling of the physical processes inside the Sun. The book is recommended for graduate students and specialists in solar physics, astrophysics, and geophysics.