Chapter 1: The three-dimensional nature of calixarenes and their applications in sensors are reviewed. Chapter 2: Molecular mechanics was used to model the geometry of a sodium calixarene complex formed with a 1,3-alternate conformation. Partial charges were assigned to the calixarene ligands by a variety of methods. These included various semi-empirical methods (AMI, PM3 and MNDO) arbitrarily assigning formal charges to the ions and carrying out semi-empirical calculations on the full ligand:complex. An investigation into the effect of placing the cation in different starting positions was undertaken in the case of this complex also. The effect of using ligand partial charges calculated with the PM3 and MNDO semi-empirical methods is commented upon. Chapters 3 and 4: A series of calixarene phosphine oxides were used to construct ion-selective electrodes, (i.e., potentiometric sensors) and were found to be selective for calcium in the presence of a range of Group 1 and Group 2 interfering ions. Estimations of the characteristics of these electrodes (the cell constant, the Nemstian Slope Factor and potentiometric selectivity coefficients) were obtained by simple but rigorous procedure, developed for a potassium-selective valinomycin ionselective electrode. The procedure involved fitting the experimental data to a mathematical model by non-linear analysis. In this method, the interfering ion concentration is kept constant and the primary ion concentration is varied by means of a series of small volume ‘spikes’, added to a single solution, with the electrodes continually in contact, and measuring the cell potential after each addition. Monte Carlo conformational searches with molecular mechanics geometry optimisation gave very clear and positive explanations for the selectivity several calixarenes for several Group 1 and Group 2 ion, while NMR spectroscopic studies supported the conclusions drawn from the modelling studies about changes in conformation upon complexation with metal ions.