Nowadays, offshore wind energy industry is developing exponentially, due to the significant contribution of the North Sea wind turbines energy production to the total consumed energy in Europe. Given that the EU's energy target is to increase the share of renewable energy by 2020, there is a great potential of the offshore wind energy applications towards this direction. In order to achieve this, the support of scientific research is crucial. Monopiles have been by far the most common support structure for offshore wind turbine, nowadays becoming applicable also for complex site conditions. Monopiles foundations consist of hollow, thin-walled piles, and they are generally subjected to lateral loading of monoonic and cyclic nature. The standard design methods are not always suitable for the optimized design of this type of foundation. Especially design of the large monopile under cyclic loading is still demanding the support of scientific research. Therefore, the main objective of this research work is to study the soil-pile interaction problem of offshore wind monopiles under both monotonic and cyclic lateral loading. The work focuses particularly on monopiles embedded in clayey soils. Special attention is given to the behaviour of natural clays, by proposing a novel constitutive law for natural clay under monotonic and cyclic loading. Furthermore, the constitutive law is employed for numerical analyses of the laterally loaded monopiles problem. ; FRS-FNRS FRIA