The use of wireless sensor networks (WSNs) has grown enormously in the last decade, pointing out the crucial need for scalable and energy-efficient routing and data gathering and aggregation protocols in corresponding large-scale environments. Hierarchical clustering protocols (as opposed to direct single-tier communication schemes) have extensively been used toward the above directions. Moreover, they can greatly contribute to overall system scalability, lifetime, and energy efficiency. In this chapter the state of the art in corresponding hierarchical clustering approaches for large-scale WSN environments is presented. The need for clustering in WSNs is first motivated and a brief description of the implied hierarchical network pattern is given. The basic advantages, objectives, and design challenges are also briefly explored. A set of appropriate taxonomy parameters as well as a global classification scheme is then introduced. In the main body of the chapter the most significant of the existing WSN clustering algorithms are concisely presented and commented according to the previously stated parameters and classification scheme. The chapter is concluded by stating some general remarks as well as some open research issues in the field.