Runoff and soil erosion are serious and widespread land degradation problems in arid and semi-arid regions. Soils in these regions are characterised by relatively high levels of salinity and sodicity, and low structural stability. One means to increase the stability of soil structure is the use of synthetic polymers as soil conditioners. Polymers consist of repeated small identical units (monomers) coupled together to form extended chains. The present review paper addresses the properties of synthetic polymers, their interactions with clays and soil particles, and their effects on infiltration rates, runoff, and soil erosion in arid and semi-arid soils exposed to rainfall and sprinkler irrigation. Because polymers increase the stability of the soil structure, they reduce the tendency of soils to form seals, thereby preventing decline in infiltration rates, reducing runoff and soil losses. The beneficial effects of various polymers with different molecular weights, charge types, and densities applied over a wide range of conditions, i.e. dryland and irrigated soils, soils of varying physical and chemical properties, and rainfall with different kinetic energies, are reviewed in the present manuscript. The addition of small amounts of the polymers, with their low cost and their high efficiency in preventing seal formation, infiltration rate reduction, and runoff and soil loss increases, suggest that the polymers can be considered as successful stabilising agents for soils that are sensitive to seal formation. However, further study on handling and field application of the synthetic polymers is needed.