The largest Solar Electric Generation System (SEGS) currently in operation in Kramer Junction, Southern California, uses a parabolic trough solar collector and synthetic oil in the collector loop to transfer thermal energy to a Rankine cycle turbine via a heat exchanger. To improve performance and reduce costs, future designs propose direct steam generation (DSG) in the SEGS's collector field. Thus evaluation of the thermohydrodynamic performance of the DSG collector is a major task that has to be completed before these systems can be considered for commercial application. In this thesis thermal and hydrodynamic models of a DSG collector are developed to investigate collector performance at steady and transient state operation. The thermal model is developed by modelling radiation loss, convection loss, and conduction loss from the absorber tube. The hydrodynamic model consists of two main parts: flow pattern analysis, and pressure drop calculations. Different flow patterns that may occur in the solar absorber tube are considered in the model. Pressure drop correlations are validated with water-steam test data at conditions similar to the DSG collector. The thermal and hydrodynamic models are integrated into a thermohydrodynamic model to evaluate the steady-state performance of the DSG collector. The thermo-hydrodynamic model is combined with a model of the other components of a SEGS to study its performance with different collector-field and power-house arrangements. To find the effect of collector inclination on the SEGS overall performance, the SEGS performance is studied with a horizontal and an inclined DSG collector field by considering pressure drop in the collector field piping network. The appropriate spacing of collector arrays in the field is calculated by considering the shading between collectors. The annual performance of SEGS is evaluated using the hourly radiation data of different sites in Australia. A mathematical model of the transient operation of a solar absorber tube is developed by applying equations of conservation of mass, momentum and energy. The mathematical model is discretised in space and time and solved by an explicit finite difference technique. The transient model is used to study the collector operation during sudden changes in radiation and typical radiation conditions.