The philosophy of deformations: introductory remarks and a guide to this volume.- A. Deformations of algebras.- Algebraic cohomology and deformation theory.- Perturbations of Lie algebra structures.- Cohomology of current Lie algebras.- An example of formal deformations of Lie algebras.- On the rigidity of solvable Lie algebras.- Triangular algebras.- B. Perturbations of algebras in functional analysis and operator theory.- Deformation theory for algebras of analytic functions.- Close operator algebras.- Perturbations of function algebras.- Perturbations of multiplication and homomorphisms.- C. Deformations and moduli in geometry and differential equations, and algebras.- Local isoformal deformation theory for meromorphic differential equations near an irregular singularity.- Geometric and Lie-theoretic principles in pure and applied deformation theory.- Complexes of differential operators and symmetric spaces.- Deformation theory of geometric and algebraic structures.- Some rigidity results in the deformation theory of symmetric spaces.- D. Deformations of algebras and mathematical and quantum physics.- Applications of the deformations of the algebraic structures to geometry and mathematical physics.- Formal deformations of the Poisson Lie algebra of a symplectic manifold and star-products. Existence, equivalence, derivations.- Invariant deformations of the Poisson Lie algebra of a symplectic manifold and star-products.- E. Deformations elsewhere.- A remarkable matrix.- Deformation stability of periodic and quasi periodic motion in dissipative systems.- List of participants.