Global wind electricity-generating capacity increased by 28.7 percent in 2008 to 120,798 Gigawatts. This represents a twelve-fold increase from a decade ago, when world wind-generating capacity stood at less than 5 GW . With wind becoming a key part of the electrical mix in Denmark (20% with 3.1 GW), Spain (8% with 10 GW), and Germany (6% with 18.4 GW), wind turbine reliability is having a bigger effect on overall electrical grid system performance and reliability . This shows the impact of faults and downtime on the reliability of wind turbine especially for offshore wind farms which although are some of the most environmentally friendly and efficient methods to generate electricity in the world. However, the maintenance costs are high because of their remote location. This can amount to as much as 25 to 30% of the total energy production . The aim of this paper is to present an overview of fault detection in wind turbines, study and analyze the faults and their root-causes. The paper also explores different techniques used in early fault detection to form base information for future work to build a general fault diagnostic scheme for wind turbines.