Monopile pile foundations are the most commonly used support system for offshore wind turbines in Europe. These foundations are subject to lateral loading, which is cyclic in nature, with the loading rate and load eccentricity varying with time. Under extreme conditions, such as the emergency stop of the turbine, yawing or ship impact, transient loading is applied to the monopile, and a high loading rate is induced in the surrounding soil. With the increase in the soil loading rate, soil strength can be enhanced, leading to a higher pile capacity. These beneficial phenomena, known as rate effects, have been principally reported in element testing research and observed in laterally-loaded monopile testing in glacial till and dense marine sand in the Pile Soil Analysis (PISA) project. However, current design methods have yet to include rate effects, and more data are required to quantify rate effects to inform optimised monopile design. This thesis systematically explores rate effects on the response of monopiles to lateral loading through laboratory-floor model testing in artificial dry sand and naturally-formed saturated stiff clay. First, a new loading apparatus for model pile testing was developed. It features a high-speed loading system which can apply rapid loading to the pile, and the height of the loading point is variable to investigate load eccentricity effects. Next, monotonic and uni-directional one-way cyclic lateral loading tests were conducted at different loading rates. In addition to rate effects, model piles were tested for various vertical loading and load eccentricities to study combined load effects. Finally, rate effects were quantified based on test data. It was found that rate effects are evident in monotonic and cyclic lateral loading in stiff clay (approximately 10% increase in capacity per log-10 cycle of the pile rotation rate). In contrast, rate dependency in dry sand appears to be negligible. Together with the experimental work, a new practical monopile design model is developed under the hyperplasticity framework in this thesis. The model captures the coupled moment-horizontal load response using a yield surface approach. In addition to lateral soil resistance, base resistance can be implemented in the model. Furthermore, rate effects for monopiles can be modelled by incorporating rate-dependent functions. To simulate pile response to cyclic loading, the Hyperplastic Accelerated Ratcheting Model (HARM) can also be integrated into the design model. Theoretical modelling results are presented in this thesis to demonstrate the performance of the model.