Psi, the unknown factor(s) in instances of apparently anomalous interaction between an organism and its environment, has long defied attempts to be usefully incorporated in a theoretical framework. By considering the different levels - physical, physiological and psychological - at which psi phenomena may be viewed as having an effect, a system theoretic approach is taken to model the way in which two systems may interact. The physical level is first considered, making the assumption that there exists an energetic psi signal. Inferring the required properties of such a signal from the types of system successfully used in psi experiments, it is proposed that the signal may be seen as acting to modify fluctuations in the electromagnetic zero-point field, its precise characteristics being determined by the activity of the electrodynamic system which generated it. A typical target system is simulated, and the presence of a perturbation - the psi signal - is shown to parallel the type of effect seen in empirical data. An experiment looking for possible distance attenuation effects using a system predicted to be sensitive to the proposed signal type is detailed. Expanding on this basic concept, a more detailed study is made of different systems and the possible types of emissive activity they may undergo. The same systems are then considered for their receptive properties. It appears that the systems most capable of detecting a psi signal are biological cells, with the site of primary interaction being the cell membranes, the semi-conductor properties of which are compared to non-biological systems. Two experiments are reported, one looking at human and the other at non-biological sensitivity to electromagnetic fields, these being considered to be detected in an analogous manner to a psi signal. Similarities in the electromagnetically modified activity of the different systems were found. Next the psychophysiological factors which would better enable a human to make use of psi signals are considered.