Applications should influence and mould the technology they are designed for. This is especially true for wireless sensor networks, a trend that has been somewhat driven by technology. This chapter begins by providing a variety of application scenarios and putting the concept of wireless sensor networks into a larger context. These application scenarios will eventually be utilised to motivate specific technical requirements. Additionally, it generalises from particular examples to different application kinds or classes.The specific difficulties associated with various application categories are then described, along with the reasons why present technology is unable to address them.