Fiber lasers have a number of distinct advantages over their more conventional solid state laser alternatives. These advantages include size, reliability, wavelength selectivity, heat dissipation, wall plug efficiency and operational cost. Furthermore they can be operated without the need for active cooling or optical alignment. Consequently the market for these more traditional laser sources are beginning to be eroded by the emergence of fiber lasers. In 1999 high power fiber lasers became a reality, with the world's first single-mode fiber laser exhibiting in excess of 100W cw output. However it was soon recognized that conventional small core, high NA fiber designs were not appropriate to applications requiring further scaling of the output power. More specifically it was found that the maximum achievable output power in such fibers were restricted by a fundamental susceptibility to optical nonlinearities, including stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and self-phase modulation. In order to overcome the limitations imposed by these parasitic nonlinear processes, it has been necessary to develop fibers with high rare-earth dopant concentrations in relatively large core, low numerical aperture fibers. These so-called large mode area (LMA) fibers are directly responsible for the recent explosion in demonstrated diffraction-limited beam quality output powers, now approaching the kW-level from a single fiber. To further scale the output power it is necessary to combine the output of several fiber lasers. Indeed, for a number of industrial and military applications it is desirable to scale the total output power to between several and hundreds of kW's. It is therefore advantageous to be able to coherently combine the beams from multiple fibers and this it turns makes it desirable for the fiber to also be polarization maintaining. This provides yet another layer of complexity to the fiber design but such fibers are now a commercial reality. In this paper we review the recent and ongoing advantages in fiber design that is facilitating the development and production of lasers and amplifiers with ever increasing output powers.