Sensor node localization is one of the most challenging problems in the wireless sensor network field. Although many excellent works have been done for the sensor node positioning issue, it is still an open problem. This chapter tries to give an comprehensive introduction about the sensor node localization in WSN. Starting from the method taxonomy based on the feature of those localization solutions, all three categories including (i) ranging-based, (ii) ranging-free and (iii) event-driven localization are discussed. In addition, the basic ideas for thirteen well-known sensor node localization papers belonging to diverse categories are discussed for giving solid examples. At the end of the chapter, all techniques are summarized, compared and commented.