A number of investigations, led over many years1-6, all indicate that defects, viz. focal conic domains, screw and edge dislocations, play an important role in the plastic behavior of smectics under shear. Although some of the mechanisms in play have been described2,6, the situation is far from being satisfactory, because of a lack of in-situ observations. In this work, we bring some new elements to the question of the interactions between dislocations at a microscopic level. We have developed an experimental setup designed to observe the motion of microscopic defects under polarized light microscope in a homeotropic slab of a SmA material submitted to a strain7. The deformation of the cell and its temperature are controlled and the motion is followed by video. The following results have been reached and discussed in a theoretical framework. (a) The existence of several regimes for the motion of dislocations and their transitions (b) The existence of a yield stress due to the depinning of the mobile edge dislocations dislocations from the fixed screw dislocations; the associated forces, energies, and shapes of the moving lines are calculated; (c) The role of the heterogeneous/ homogeneous nucleation of dislocations under strain We conclude that the interactions between defects actually rule the plastic deformation of the confined smectic and we discuss the origin of observed interactions. References : 1 Bartolino and G. Durand, Phys. Rev. Lett., 1977, 39, 1346. 2 R.G. Horn and M. Kléman,. Ann. de Phys. 1978, 3, 229. 3 L. Bourdon, M. Kléman, L. Lejcek and D. Taupin, J. de Phys. 1981, 42, 261. 4 P. Oswald and M. Kléman, J. de Phys. Lettres 1984, 45, L319. 5 R. A. Herke, N. A. Clark, and M. Handshy, Science 1995, 267, 651. R. A. Herke, N. A. Clark, and M. Handshy, Phys.Rev. E 1997, 56, 3028. 6 C. Meyer, S. Asnacios, and M. Kleman, Eur. Phys. J. E, 2001, 6, 245. 7 I.Lelidis, M.Kleman, and J.L.Martin, Mol. Cryst. Liq. Cryst., 2000 , 351,187.