The paper presents the analysis of the effect of Fluid-Structure Interaction (FSI) occurring during hydraulic transients in pipe coils, in particular the main developments and findings. The research work comprises the development of mathematical models, their numerical implementation and their validation as compared with experimental evidence. The aim is to model the behavior of toric pipes during hydraulic transients considering both axial stress waves in the pipe-wall and fluid conservation principles. Three FSI mechanisms are taken into account: the shear stress generated between the fluid and the pipe-wall, the axial movement of the pipe induced by its radial deformation and the pipe movement generated by an imbalance of forces at junctions and boundaries. Hence, Poisson, junction and friction coupling are implemented. To describe the coil structural behavior two conceptual models are developed: the first representing the coil as a straight pipe with a moving valve, and the second assuming independent axial deformation in each coil ring. Although the first approach allows an easier generalization of the method, the second model describes more accurately the FSI problem in the pipe coil as experimentally observed. The paper novelty is the identification and description of a FSI phenomenon occurring in coils by means of a four-equation model.