Novel passive array processing algorithms are proposed which are related to the localisation of the source and multipath reflectors of a desired signal in the presence of co-channel interference under the narrowband and wideband assumptions. An important limitation of localisation approaches which are based on the Line-of-Sight multipath of the desired signal is the fact they are unsuccessful in the presence of multipath. For this reason, in this thesis a localisation preprocessing algorithm is proposed which removes all the multipaths from the signal received by the array except for the Line-of-Sight one, thus enabling Line-of-Sight based localisation approaches to operate in the presence of multipath. In addition, a novel algorithm is proposed for the localisation of signal sources and multipath reflectors lying inside the near-far field of a large or small aperture array. The proposed approach expresses the unknown path delay as a function of path range and this allows the source and multipath reflectors of the desired signal to be unambiguously localised in the presence of Multiple-Access-Interference or jammers. Furthermore, a localisation-assisted beamforming algorithm is proposed for large aperture arrays consisting of a number of small aperture arrays which don't receive the same number of signals (multipaths). For each multipath of the desired signal, the path delay, range and direction of arrival are estimated and then these estimated channel parameters are used to construct the proposed beamforming weights. Computer simulation studies which investigate and evaluate the performance of the proposed algorithms are presented throughout the chapters of this thesis.