The thesis deals with systems consisting of marine cables and subsea units. Such systems have wide applications in offshore subsea operations. After a general introduction, the thesis sets out to analyse both the static and dynamic behaviours of the system under various environmental and operational conditions. It endeavours to pursue a fundamental approach in order to reveal the basic characteristics of the system, in addition to developing numerical algorithms for predicting performance. The analysis of behaviour of marine cables consists of the following parts: Statics A semi-analytic approach is developed to predict the equilibrium configurations of marine cables. One-dimensional dynamics Using a coordinate transformation, the method can predict the unsteady dynamic behaviour of systems where the length of cable varies. Two-dimensional dynamics The methodology adopted in the one-dimension analysis is extended to a more general case. Three-dimensional dynamics An alternative approach based upon a lumped mass model is developed. Mathematical analysis reveals many interesting characteristics of the model. By applying modern control theory, a novel heave compensation mechanism is developed for marine systems of cables and subsea units. This mechanism involves an actively controlled winch system. A framework of optimal stochastic control is outlined for integrating all the elements of surface supported subsea operations. The thesis presents a variety of numerical examples in domenstrating the validity of the approaches adopted, along with discussions. Further developments are also recommended.