Despite the general recession in the global economy and the collapse of the optical telecommunication market, research within specialty fibers is thriving. This is, more than anything else, due to the technology transition from standard all-glass fibers to photonic crystal fibers, which, instead of doping, use a microstructure of air and glass to obtain a refractive index difference between the core and the cladding. This air/glass microstructure lends the photonic crystal fibers a range of unique and highly usable properties, which are very different from those found in solid standard fibers. The freedom to design the dispersion profile of the fibers is much larger and it is possible to create fibers, which support only a single spatial mode, regardless of wavelength. In comparison, the standard dispersion-shifted fibers are limited by a much lower index-contrast between the core and the cladding, leading to reduced mode confinement and dispersion flexibility. In this thesis, we treat the nonlinear photonic crystal fiber – a special sub-class of photonic crystal fibers, the core of which has a diameter comparable to the wavelength of the light guided in the fiber. The small core results in a large nonlinear coefficient and in various applications, it is therefore possible to reduce the required fiber lengths quite dramatically, leading to increased stability and efficiency. Furthermore, it is possible to design these fibers with zero-dispersion at previously unreachable wavelengths, paving the way for completely new applications, especially in and near the visible wavelength region. One such application is supercontinuum generation. Supercontinuum generation is extreme broadening of pulses in a nonlinear medium (in this case a small-core fiber), and depending on the dispersion of the fiber, it is possible to tailor the broadening to generate a spectrum with a special width or wavelength coverage. Supercontinuum generation is just one of the many applications for the nonlinear photonic crystal fiber. They are also highly suitable for use in telecommunication demultiplexers and various applications based on four-wave mixing. In addition, the broad supercontinua have a range of applications within biomedicine, telecommunication and metrology. The special structure of photonic crystal fibers opens up the possibility, in a simple way, to create polarization-maintaining fibers without the use of the stress elements incorporated in standard polarization-maintaining fibers. The needed birefringence is introduced by breaking the six-fold symmetry around the core, and introducing a structure with two-fold symmetry. In this way, it is possible to create fibers, which are not only polarization maintaining, but also hold the unique dispersion properties of the normal nonlinear photonic crystal fibers. The fact that birefringence is easily introduced in these fibers is not without problems as small asymmetries, unintentionally created during fabrication, can also cause birefringence. Unlike the deliberately introduced birefringence, the fabrication-induced birefringence fluctuates along the length of the fiber, making them unsuitable for e.g. four-wave mixing. Furthermore, the birefringence is wavelength dependent, which affects the dispersion and especially the zero-dispersion wavelength. It is therefore important that the fibers are either polarization maintaining or have a low level of birefringence, where the latter can only be achieved by ensuring a high degree of transverse symmetry in the structure (typically better than one percent). Photonic crystal fibers were demonstrated for the first time in 1996, and are today on their way to become the dominating technology within the specialty fiber field. Whether they will replace the standard fiber in the more traditional areas like telecommunication transmission, is not yet clear, but the nonlinear photonic crystal fibers are here to stay.