Most papers in this book were presented at the SOHO-9 Workshop ''Helioseismic Diagnostics of Solar Convection and Activity'', held on 12-15 July 1999 at Stanford University, California, U.S.A. Some papers which were not presented at the work shop have been added, after authors had been invited in Solar News to contribute to this book. All papers submitted to this book have been refereed, and only those of high scientific quality were accepted for publication. The book focuses on the recent advances in our understanding of solar con vection and activity, and on 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 analy sis, acoustic imaging and holography), intensively developed in the past few years, have provided promising results on the deep structure of large-scale convection and flows, emerging active regions, and sunspots. Along with the traditional ''global'' helioseismology based on frequencies of resonant oscillation modes, the local-area techniques have become increasingly important for studying the solar interior and the mechanisms of solar activity. The new high-resolution helioseismology projects ''Solar Oscillation Investiga tionlMichelson Doppler Imager'' (SOIlMDI) on board SOHO, and ground-based ''Global Oscillation Network Group'' (GONG) have provided a tremendous amount of solar oscillation data. Extracting from these data the information about the in ternal properties and dynamics of the Sun, and using this information in theoretical models and space weather forecasts are new challenges in solar physics.
