The photonic crystal fiber (PCF) is a novel single-material optical waveguide realized by an arrangement of air-holes running along the full length of the fiber. Since the proposal of the PCF in 1996, the technology has developed into being a well-established area of research and commercialisation. The work presented in this thesis deals with the optical properties of large-mode area PCFs for which the mode-field diameter, typically, is an order of magnitude larger than the free-space optical wavelength. Special emphasis is put on the description of relevant mechanisms of attenuation in these fibers. This includes design guidelines for optimising the robustness of single-mode fibers and expressions for predicting the mode-field diameter and dispersion properties for a given choice of structural parameters. Microdeformation induced attenuation is addressed and shown to play a critical role for the limitation of the available bandwidth. Screening of such deformations is explained from a theoretical point of view and experimentally verified. Furthermore, macro-bending induced attenuation is addressed and an analytical expression predicting the spectral dependency is proposed. The expression can be evaluated based on the structural parameters and predicted data agree within 100 nm with experimental observations. Attenuation related to the fabrication process is described including central issues such as hydroxyl contamination, defect centers in the glass, along with contamination and roughness of surfaces in the perform. As a result of fabrication optimisation, a single-mode fiber with an effective area of 130 m2 and attenuation of 0.48 dB/km at the 1550 nm wavelength is reported. Based on the general consideration of the introducing chapters, 5 different examples of large-mode area PCFs are presented. The first is a large-mode area fiber optimised for visible light applications. The second is a fiber optimised for the telecommunication band realizing a nonlinear effective area 5 times larger than state of the art conventional fibers. Two examples of alternative designs are demonstrated addressing the core and the cladding region, respectively. The last of the five examples is the first polarization-maintaining large-mode area PCF to be reported. The fiber design combines the benefits of the PCF with utilization of stress applying parts for the realization of material birefringence in the core region of the fiber. The result is a large-mode area fiber which is both endlessly single-mode and endlessly birefringent.