This book presents a collection of lectures given during the academic year 2000-2001 at the Instituto de Acústica CSIC to commerate the 25th Anniversary of its official foundation as a Research Institute. They represent a good selection of the topics of current interest in emerging fields of Acoustics (Buildings Acoustics and Active Noise Control), Ultrasonic (Medical Therapy, Transducers and High Power Untrasound), Physical Acoustic (Shallow Water Acoustic and Nonlinear Acoustic) and Numerical Acoustic. The contributions generally combine elements of a review with recently obtained results, giving the reader an insight into the up-to date status.