Photonic crystals are periodic dielectric structures, where the periodicity varies on the wavelength scale. Analogous to electrons in semiconductors, the photon propagation can be described using a band structure in which transmission bands are separated by bandgaps, energy ranges at which light cannot exist inside the photonic crystals. This analogy suggests that photonic crystals may be suitable to fabricate the components needed for integrated optics. As yet most of the research done on photonic crystals has been focussed on using these bandgaps. However, many novel properties of light propagating in the transmission bands may also serve this purpose. This thesis studies theoretically and experimentally the light propagation in two dimensional planar photonic crystals with the aim of creating new components for integrated optics. A theoretical model based on Fourier analysis of electromagnetic Bloch waves was developed to describe light propagation in photonic crystals. This model gives an intuitive understanding of the novel phenomena observed in the transmission bands of the photonic crystal: negative refraction, auto-collimation and super-dispersion. This new approach clarifies the fundamental physics of these phenomena enabling their advantages and disadvantages to be easily evaluated for new optical functions; particular attention is paid to auto-collimation. The influence of the structural properties of the photonic crystal (lattice type, filling factor, etc.) is also studied and has been used to create two integrated optic components: a light condenser and a wavelength demultiplexer. The theoretical predictions have been tested experimentally on two dimensional planar photonic crystals fabricated in heterostructure GaAs/AlxGa1-xAs. The fabrication of these photonic crystals was the first experimental work undertaken in this thesis. Advanced microfabrication techniques enabled the fabrication of planar photonic close to those of the state-of-the-art. The optical properties of the fabricated photonic crystals fabricated were studied using the internal light source technique. This method uses photoluminescence generated by quantum wells or quantum dots as an internal light source to probe the optical properties of the photonic crystals. Initially the experimental setup was designed to study the bandgaps. However after some modifications the transmission bands could also be investigated. Another experimental improvement made was the introduction of a technique using coupled waveguides which reduces the losses by a factor of 10 and allowed the light in the photonic crystals to be studied over long distances. Adding a spatially resolved vertical detection, enabled the analysis of the light diffracted out of the plane in order to measure the propagation losses and to map the electromagnetic field. These two modifications were used to study components for integrated optics based on photonic crystals: waveguides and auto-collimators.