This chapter is a primer on WiFi-fingerprint-based indoor positioning systems (IPSs). It assumes no prior knowledge of the systems or how they work. We first elaborate the necessity of WiFi positioning technology. Further, three basic categories of WiFi positioning methods are illustrated, i.e., deterministic methods, probabilistic methods, and machine learning methods. Next, a brief introduction to some sub-issues in fingerprint-based positioning is presented, e.g., device heterogeneity and positioning time. Lastly, multi-sensors assisted fingerprint positioning technology is introduced to improve positioning accuracy.