Emerging offshore wind farms, capable of massive-scale energy production, have directed research activities toward wind power technologies, well-suited for offshore applications. Increasing the size of offshore wind farms imposes challenges regarding feasibility of conventional ac power transmission and collector systems. The main reasons are the high charging current of ac submarine cables and increasing system losses. Recent studies have identified pure dc systems - including dc transmission and collector systems- as a promising solution for grid integration of distant offshore wind farms. Using a dc system overcomes power losses and reactive power requirements which conventionally associate with submarine power cables employed in ac systems. This thesis investigates pure dc systems for offshore applications and proposes a modeling and control approach besides an appropriate topology for the collector system of an offshore wind farm. The collector system of an offshore wind farm consists of all equipment within the wind farm to a collection point, including the wind turbine-generators, transformers, power electronics and all existing cable connections. First, through investigation of different wind energy systems, a dc-based offshore wind farm has been considered, consisting of permanent magnet synchronous generators, diode-bridge rectifiers and dc/dc converters. Such integration offers a robust structure well-suited for offshore wind power applications. A simplified yet accurate model for a wind power unit is proposed and validated, which is useful for controller design to capture the maximum wind power of the unit. A control technique is also presented and its performance has been investigated based on sensitivity analysis and time-domain simulation of a study system. The results of analysis show an accurate tracking performance with less than 1% error when the wind speed varies about +/-25% of its rated value. The sensitivity analysis also confirms the robustness of the control system against model uncertainties which can arise from the aging phenomenon of the units. Secondly, a matrix interconnected topology has been proposed and investigated for collector system of an offshore wind farm. The topology is based on the series-parallel layout in which some switches are located between adjacent branches. The possibility to change the system structure using the inter-branch switches mitigates the overvoltage problem of the units on failure occurrences, providing maximum available power for the whole wind farm. This leads to considerable increase in the system reliability and efficiency. Based on failure coverage capability, the operating area of matrix interconnected method has been evaluated and compared to that of the short circuit method. The study shows that increasing the number of branches augmented to the faulty branch extends the matrix method operating area. This analysis also suggests a criterion for the branch number in a dc collector system in order to have a full coverage of failure occurrences by matrix interconnected method. This criterion might be useful in collector system design purposes. Finally, a study system, consisting of 12 wind turbine-generator units, has been simulated and investigated for different failure occasions. The dc switch costs and the additional control complexities are counted as the drawbacks of the matrix interconnected topology Keywords: Collector systems, DC power systems, Offshore wind farm, Pure dc systems, Wind energy systems, Wind-turbine generator.