In this chapter, we will discuss the concepts and principles of the discrete Zak transform (DZT)-based orthogonal time-frequency space (OTFS) for doubly dispersive channels. OTFS is a two-dimensional modulation technique that translates a rapidly time-varying channel into a quasi-static, time-invariant channel by modulating the information symbols in the delay-Doppler (DD) domain. The DD domain originates from radar communication literature, where each target is identified by its delay- and velocity-induced Doppler. Therefore, we will first introduce some radar concepts and how they address the doubly dispersive nature of real-time channels. The core of the DZT lies in the DD domain. Hence, before formally introducing the DZT, we will explore features of the DD domain, including the concept of non-fading, crystallization, etc. Then, we will proceed with the definition of the DZT, its properties, and Zak basis functions. Following that, we will discuss how OTFS is constructed based on the DZT, its matrix representation, and also a comparison between pre-coded OTFS and DZT-based OTFS. Finally, we will conclude this chapter with some applications.