This volume contains the proceedings of the 21st International Congress of Theoretical and Applied Mechanics, ICTAM04, held in Warsaw, in August 2004. Full texts of 27 invited lectures are included. The book captures a snapshot view of the state-of-the-art in the field of contemporary mechanics and will be invaluable to engineers and scientists from a variety of disciplines with interest in the mechanical sciences. The importance of the influence of contemporary mechanics on other branches of sciences becomes evident by browsing through over 60 areas of interest selected as subjects of mini-symposia and pre-nominated sessions. The book gives clear evidence that '...the progress we have achieved together definitely places mechanics on one of the very top locations in the hierarchy of modern research disciplines with tremendous impact on both our perception of the physical world and the means to implement new technologies so much improving the quality of our life.' (M. Kleiber, Opening Speech).