The application to Biology of the methodologies developed in Physics is attracting an increasing interest from the scientific community. It has led to the emergence of a new interdisciplinary field, called Physical Biology, with the aim of reaching a better understanding of the biological mechanisms at molecular and cellular levels. Statistical Mechanics in particular plays an important role in the development of this new field.

For this reason, the XXth session of the famous Sitges Conference on Statistical Physics was dedicated to "Physical Biology: from Molecular Interactions to Cellular Behavior." As is by now tradition, a number of lectures were subsequently selected, expanded and updated for publication as lecture notes, so as to provide both a state-of-the-art introduction and overview to a number of subjects of broader interest and to favor the interchange and cross-fertilization of ideas between biologists and physicists.

The present volume focuses on three main subtopics (biological water, protein solutions as well as transport and replication), presenting for each of them the on-going debates on recent results. The role of water in biological processes, the mechanisms of protein folding, the phases and cooperative effects in biological solutions, the thermodynamic description of replication, transport and neural activity, all are subjects that are revised in this volume, based on new experiments and new theoretical interpretations.

Networks can provide a useful model and graphic image useful for the description of a wide variety of web-like structures in the physical and man-made realms, e.g. protein networks, food webs and the Internet. The contributions gathered in the present volume provide both an introduction to, and an overview of, the multifaceted phenomenology of complex networks. Statistical Mechanics of Complex Networks also provides a state-of-the-art picture of current theoretical methods and approaches.

This volume gathers a number of selected contributions from the XIX Sitges Conference on "Jamming, Yielding, and Irreversible Deformation in C- densed Matter", held at Sitges (Barcelona, Spain) from 14-18 June, 2004. The contributions collected in this volume provide a general overview of the "state of the art", and of the recent developments, in the ?elds of material yield and irreversible deformation in di?erent physical systems, which are of great interest within the realm of Condensed Matter Physics. The Conference was sponsored by several institutions that generously provided ?nancial support: Ministerio de Educaci' on y Ciencia of the Sp- ish Government, AGAUR of the Generalitat de Catalunya, Universitat de Barcelona, and the Centre Especial de Recerca (CER) F' ?sica de Sistemes Complexos. As in former editions of the Conference, the city of Sitges allowed us to use the beautiful Palau Maricel as the lecture hall. We thank them for their kind hospitality. We are also very grateful to all those who collaborated in organizing the event: M. Naspreda, A. P' erez-Madrid, R. Pastor-Satorras, and S. Zapperi. Finally, we wish to express our gratitude to all the speakers and parti- pants in the Conference, who contributed with high scienti?c level presen- tions, and created a very pleasant atmosphere.

This volume gathers a number of selected contributions from the XIX Sitges Conference on "Jamming, Yielding, and Irreversible Deformation in C- densed Matter", held at Sitges (Barcelona, Spain) from 14-18 June, 2004. The contributions collected in this volume provide a general overview of the "state of the art", and of the recent developments, in the ?elds of material yield and irreversible deformation in di?erent physical systems, which are of great interest within the realm of Condensed Matter Physics. The Conference was sponsored by several institutions that generously provided ?nancial support: Ministerio de Educaci' on y Ciencia of the Sp- ish Government, AGAUR of the Generalitat de Catalunya, Universitat de Barcelona, and the Centre Especial de Recerca (CER) F' ?sica de Sistemes Complexos. As in former editions of the Conference, the city of Sitges allowed us to use the beautiful Palau Maricel as the lecture hall. We thank them for their kind hospitality. We are also very grateful to all those who collaborated in organizing the event: M. Naspreda, A. P' erez-Madrid, R. Pastor-Satorras, and S. Zapperi. Finally, we wish to express our gratitude to all the speakers and parti- pants in the Conference, who contributed with high scienti?c level presen- tions, and created a very pleasant atmosphere.

For the first time this subject, including many systems of interest in Condensed Matter Physics, is treated in an unified way. Complexity emerges as one of the main ingredients dictating the collective behaviour of many systems. Glassy systems constitute one of the most interesting fields of Condensed Matter Physics for which also a considerable amount of experimental data and industial applications have been collected during the last twenty years. Systems exhibiting glassy behaviour are for example: real glasses, spin glasses, vortex flasses in superconductors, protein folding, etc. In this book the reader can see how the present theoretical understanding of these subjects is based on similar techniques and approaches hopefully allowing to develop a unifying structure that underlies the physical mechanism.

The application to Biology of the methodologies developed in Physics is attracting an increasing interest from the scientific community. It has led to the emergence of a new interdisciplinary field, called Physical Biology, with the aim of reaching a better understanding of the biological mechanisms at molecular and cellular levels. Statistical Mechanics in particular plays an important role in the development of this new field.

For this reason, the XXth session of the famous Sitges Conference on Statistical Physics was dedicated to "Physical Biology: from Molecular Interactions to Cellular Behavior." As is by now tradition, a number of lectures were subsequently selected, expanded and updated for publication as lecture notes, so as to provide both a state-of-the-art introduction and overview to a number of subjects of broader interest and to favor the interchange and cross-fertilization of ideas between biologists and physicists.

The present volume focuses on three main subtopics (biological water, protein solutions as well as transport and replication), presenting for each of them the on-going debates on recent results. The role of water in biological processes, the mechanisms of protein folding, the phases and cooperative effects in biological solutions, the thermodynamic description of replication, transport and neural activity, all are subjects that are revised in this volume, based on new experiments and new theoretical interpretations.