Entanglement is perhaps the single-most important resource of quantum infor-mation theory. The first part of this t hesis deals with t he creation of optical event-ready entanglement with a specific class of optical circuits. These circuits include passive components such as beam-splitters and phase-shifters, and active components such as optical parametric down-converters and optical squeezers. Furthermore, the entangled-state preparation may be co ndit ioned on one or more det ector outcomes. In this context, I discuss the statistics of down-converters and give a quantitative comparison between realistic detectors and detector cascades, using the confidence of the detection. The outgoing states of the optical circuits can be expressed in terms of multi-dimensional Hermite polynomials. Event-ready entanglement cannot be created when t he outgoing state is conditioned on two detected photons. For six detected photons using ideal photo-detectors a scheme is known to exist. Part two of this thesis includes two applications of optical entanglem ent . First, I discuss quantum teleportation and entanglem ent swapping using down-conversion. It is shown that higher-order photon-pair production degrades the fidelity of the teleported ( or swapped) states. The interpretation of these states proved controversial, and I have attempted to settle this controversy. As a second application , quantum lit hography uses optical ('which-way') entanglement of mul-tiple photons to beat t he classical diffraction limit. Given a suitable photo-resist, t his technique results in sub-wavelength optical resolution and can be used to write features much smaller than is possible with classical lithography. I present classes of states which can be used to create patterns in one and two dimensions with sub-wavelength resolution.