This book contains the proceedings of a meeting that brought together friends and colleagues of Guy Rideau at the Universite Denis Diderot (Paris, France) in January 1995. It contains original results as well as review papers covering important domains of mathematical physics, such as modern statistical mechanics, field theory and quantum groups. The emphasis is on geometrical approaches with papers devoted to the study of symmetry groups, including applications to nonlinear differential equations, and deformation of structures, in particular deformation-quantization and quantum groups. Topics range from pure mathematics to up-to-date applications such as imaging and neuronal models.