The design flexibility and unique propagation characteristics of photonic crystal fibers (PCFs) offer many new possibilities for optical communication systems and for fiber-based devices. PCFs are usually formed by a central solid defect region surrounded by multiple air holes in a regular shape lattice. This thesis addresses theoretically four-wave mixing (FWM) in these fibers. FWM can be an important source of performance impairment for dense wavelength-division-multiplexing (WDM) systems and at the same time it can be used as an efficient technique for achieving wavelength conversion in these systems. The strong analogies between the modal behavior PCFs and that of conventional circular step-index fibers (SIFs) are used as a guideline to characterize the dispersion characteristics and modal behavior of PCFs. The starting point depends on published empirical relations that characterized the effective normalized cut-off frequencies and normalized attenuation constant of triangular PCFs. The dependences of group velocity dispersion and dispersion slope on various stricture parameters are identified in details. The analysis is supported by new derived expressions and simulation results obtained using CODUS software package. The calculated dispersion characteristics of the PCFs are used as a basis to assess FWM characteristics in these fibers. The dependence of spectrum and efficiency of FWM on PCFs structure parameters are studied in details. The results are used as a guideline to characterize parametric amplification in these fibers using OPTIWAVE software package. The simulated and calculated results are found to be in good agreement with published experimental data. The results indicate clearly that PCFs have higher FWM efficiency compared with conventional single mode fibers and this is attributed mainly 5 due to the lower effective area of the PCFs. Further the gain of a FWM-based PCF parametric amplifier is enhanced strongly when the fiber is designed with negligible group velocity dispersion and dispersion slope at the operating wavelength.