During this ph.d. work, attention has been focused on understanding and analyzing the modal behavior of micro-structured fibers. Micro-structured fibers are fibers with a complex dielectric toplogy, and offer a number of novel possibilities, compared to standard silica based optical fibers. The thesis focuses on understanding the basic mechanisms controlling the modal properties of micro-structured fibers. One important sub-class of micro-structured fibers are fibers that guide light by index effects similar to those index effects that ensure guidance of light in standard optical fibers. Such micro-structured fibers are the ones most often trated in literature concerning micro-structured fibers. These micro-structured fibers offer a whole range of novel wave guiding characteristics, including the possibility of fibers that guide only one mode irrespective of the frequency of light, and nonlinear fibers with zero dispersion wavelength well below 1300 nm. This thesis dexcribes the functionalities of these fibers, and further point to novel application areas, such as new efficient fiber amplifiers and fibers with new possibilities within dispersion management. When pointing toward novel possibilities, the thesis will attempot to offer a proof of concept, rather than an in-depth analysis, thus reflecting the present state of the art within the area of micro-structured fibers. Another important sub-class of micro-structured fibers is photonic bandgap fibers. Photonic bandgap fibers are far more complicated to manufacture, compared to micro-structured fibers that guide light by simple index effects, because of stringent requirements concerning the periodicity of the cladding structure of the fibers. The theoretical investigation of these fibers is also more complex, than the investigation of micro-structured fibers that guide light by simple index effects. However, photonic bandgap fibers offer more radical possibilities, such as core regions with an effective index that is lower than the surrounding effective cladding index one may guide light in air- and dispersion qualities that differ from both those of index guiding micro-structured fibers as well as from the dispersion properties of standard optical fibers.