This chapter will review an important topic in wind energy: wind resources and characteristics. The material covered in this chapter can be of direct use to other aspects of wind energy which are discussed in the other sections of this book. For example, knowledge of the wind characteristics at a particular site is relevant to the following topics:. Systems design -system design requires knowledge of representative average wind conditions, as well as information on the turbulent nature of the wind and extreme wind events. This information is used in the design and selection of a wind turbine intended for a particular site. . Performance evaluation -performance evaluation requires determining the expected energy productivity and cost effectiveness of a particular wind energy system based on the wind resource. . Siting -siting requirements can include the assessment or prediction of the relative desirability of candidate sites for one or more wind turbines. . Operations -operation requirements include the need for wind resource information that can be used for load management, operational procedures (such as start-up and shutdown), and the prediction of maintenance or system life.The chapter starts with a general discussion of wind resource characteristics, followed by a section on the characteristics of the atmospheric boundary layer that are directly applicable to wind energy applications. The next two sections present a number of topics that enable one to analyze wind data, make resource estimates, anticipate extreme wind speeds, and determine wind turbine power production from wind resource data, or from a limited amount of wind data (such as average wind speed). Next, a summary of available worldwide wind resource assessment data is given followed by a discussion of wind prediction and forecasting. The section after that reviews wind resource measurement techniques and instrumentation. The chapter concludes with a summary of a number of advanced topics in the area of wind resource characterization.