This chapter provides a very brief overview of PayPal’s Name-Value Pair (NVP) APIs, the primary way that you’ll interact with PayPal products throughout the remainder of this book. Since using the NVP APIs is as simple as making some HTTP requests and parsing the responses, we’ll get a Python project up and running with Google App Engine (GAE) as part of this initial overview. Future chapters all use Python-based GAE projects, so unless you’re definitely planning to port the code without running it, you should make the most of this opportunity to learn the basics. After reading this chapter, it is highly recommended that you check out and run the sample code for Tweet Relevance as introduced in Appendix A.