This study presents numerical and theoretical investigations on the bearing capacity of a single pile in sand under axisymmetrical loading conditions. An extensive literature review for the existing theories is presented. The published reports showed a wide range of discrepancies exists among previous theories developed to predict bearing capacity of a single pile in sand. A numerical pile load-testing program was carried out using finite element technique to cover a wide rang of pile geometry and sand states. Mohr-Coulomb criteria were used to model the sand behavior. Linear strain quadrilateral elements were employed to model soil and pile. A chain of slip elements was placed around the pile to model the slippage between sand and pile. The numerical model was validated against field load tests data. The numerical model was then used to analyze stresses influencing the pile behavior in sand and to establish the pile failure mechanism. The stresses and coefficient of earth pressure acting on the pile shaft were reported. A new failure mechanism was developed which varies with: pile geometry, coefficient of earth pressure, shaft roughness and angle of shearing resistance of sand.  A theoretical model was developed to predict the ultimate bearing capacity of a single pile in sand, utilizing the proposed failure mechanism. A data analysis procedure was employed to develop the new model parameters predictive formulas. An approximate method to predict the coefficient of earth pressure acting on the pile shaft was developed and used extensively in the theoretical model. A sensitivity analysis was conducted on the varying parameters of the theoretical model. The theoretical model incorporates salient features previously omitted in conventional bearing capacity theories: treating the bearing capacity of a single pile in sand under axisymmetrical conditions, adopting the punching shear failure as a principle failure mode, and accounting for the interdependence between skin friction and tip resistances. The theoretical model showed that the average unit skin and tip resistances increase, but at a lower rate below the critical depth. These findings concur with the recent research conclusions, which indicate that the average skin resistance tends to increase with depth. A computer program "G-Pile" was developed to facilitate the massive mathematical calculations of the theoretical model. The computer program "G-Pile" was used to produce data for charts of the factors needed to predict the bearing capacity of a single pile in sand. A design procedure is proposed and verified against field test results, good agreement was achieved. Recommendations are given for future research.