The Molten Salt Reactor (MSR) concept originates from the ORNL Aircraft Reactor Propulsion program, 1950-s. Two test MSRs were built and despite limited operational experience, the technology was proven to be feasible, but far from optimised. The current interest in MSR technology was stimulated in 2000, when the MSR concept was selected as one of the Gen IV reactors. Since 2000 several concepts of MSRs have been proposed worldwide by both private and governmental organisations, including liquid and solid fuel, fast and thermal, single and two- fluid MSR designs. The liquid fuel MSR concept features several advantages over a conventional solid fuel reactor (like a Pressurised Water Reactor (PWR)) such as low operating pressure, the possibility of on-line fuel reprocessing, load following and the drain tank safety mechanism. Even so, the MSR concept is the least explored of all proposed Gen IV designs. Significant obstacles, related to the analytical models remain that include closely coupled neutronics and thermal-hydraulics, accounting for the delayed neutron drift with the fuel salt and the continuous removal of gaseous fission products. This work is focused on reviewing the main differences in modelling MSRs and solid fuel reactors due to the circulating fuel, uniform burnup, on-line reprocessing, delayed neutron population dependency on the fuel salt flow rate, and proposes methodologies applicable to various MSRs using a Molten Salt Fast Reactor (MSFR) as an example. Apart from the circulating fuel MSRs the work also considers the fixed fuel Stable Salt Reactor (SSR). Design development and neutronics analysis of the SSR were carried out as part of this project. The SSR modelling highlights the principal differences in modelling a fixed fuel and a circulating fuel MSR. The work considers fuel salt reprocessing and outlines the importance of consideration of noble gas treatment such as extraction and storage. Argon is considered as a possible substitute of helium in bubbling process as a gas which is less prone to leakage. Modelling of thermal-hydraulics is performed via coupling of OpenFOAM generic solver with SERPENT neutronics code in order to facilitate further neutronics assessments. Extensions of the SERPENT neutronics code are made in order to assess the behaviour of the delayed neutron population under the influence of fuel salt circulation. A validation case of the Molten Salt Reactor Experiment (MSRE) is provided alongside with the analysis of the MSFR. Finally, the developed methodologies are applied in order to assess an alternative control mechanism that is proposed for circulating fuel MSRs.