The Workshop in Fluid Mechanics and Population Dynamics: Models, Existence Problems, Stability and Numerical Methods took place in Hadath (Lebanon), at the Laboratory of mathematics in the Doctoral School of Science and Technology of the Lebanese University, during the week September 10 - September 14, 2012 The workshop was held with the support of the Ecole Centrale de Nantes, the Laboratoire de mathématiques Jean Leray of Nantes university and the universities of Paris 13 and Poitiers, and was organized by Samer Israwi, Basam Kojok, Ayman Mourad, Mazen Saad, Zaynab Salloum and Raafat Talhouk. It was attended by about 60 PhD students and Mathematicians coming from universities from different countries, such as Algeria, France, Lebanon and Tunisia. The workshop included courses tutorials and sessions around specific themes in which participants presented their works. It was open to any scientist interested in academic or industrial applications. It was aimed at doctoral students in mathematics, as well as senior researchers. Its objective was firstly to present a synthesis of various partial differential equations involved in fluid mechanics and populations dynamics, and on the other hand to present more recent advances in research topics such as asymptotic models in fluid mechanics, homogenization and the finite volume method. All the contributions to this special volume were submitted to regular peer review, according to the rules of the journal Discrete and Continuous Dynamical Systems - Series S. We deeply thank all the authors for their high quality papers and the Editors of the journal for their interest in the topics of the workshop. We express our heartfelt thanks to the following sponsors: Ecole Central de Nantes and Laboratoire de Mathématques Jean Leray, Nantes University, represented by Mazen Saad, Université de Poitiers, represented by Alain Miranville, Université Paris 13, represented by Mikhael Balabane, Lebanese National Council for Scientific Research (CNRS) and Lebanese University. Finally, we express our sincere thanks to the Dean of the Doctoral School of Science and Technology at the Lebanese University, Professor Zeinab Saad, for her support throughout the workshop and to the French Embassy in Beirut, represented by its Scientific Counselor, Mr. Gilles Théodet.