In this paper the results of the systematical investigation of the basic effects in Nd-doped silica fibers and their application in high-power fiber-lasers will be presented. The development and manufacturing of rare-earth doped double-clad optical fibers based on silica was done at the department 'Moderne Optik' of the Institut fur Physikalische Hochtechnologie e.V. Jena (IPHT). In the last few years at the IPHT Jena rare-earth doped double-clad fibers based on silica for the application in high-power fiber-lasers were developed and characterized. The doping with Neodymium as the laseractive component was one of the most important questions which were investigated. Many samples had to be made for the determination of the optimum concentration of Nd, the optimum codoping and geometry, respectively. As a result we realized a double-clad fiber with D-shape geometry of the pump-core. This D-shape resulted from ray-tracing calculations performed at the Laserzentrum Hannover e.V. (LZH) as an optimal solution for the high conversion efficiency of the pump power. Considering these numerical results, such double-clad fibers with different diameters of the Nd-doped laser-core (multimode or single-mode) was realized at IPHT by drawing of sidepolished preforms with different compositions and geometries by preserving the cross section of the preform. In a tight cooperation with the LZH cw-output powers of more than 30 W (multimode fiber-laser) and approx. 14 W (single-mode fiber-laser), respectively, were realized in the lab. First compact and transportable 19'-devices of high-power fiber- laser with a pumping power of approx. 10 W and an output power of more than two watts were developed and realized.