This chapter comprehensively reviews the developmental anatomy and histology of the posterior longitudinal ligament (PLL). The structure of the PLL is analyzed and its function is explained in detail. Its important role in maintaining spinal stabilization is highlighted. Information regarding its embryonic origin and the contribution of the mesoderm to its formation is summarized. The chapter also addresses the clinical relevance of PLL in the contexts of trauma assessment, diagnosis of spinal disease, and surgical intervention, and emphasizes the importance of the developmental anatomy and histology of the PLL in spinal surgery. An in-depth understanding of the anatomy of the PLL is provided because this is essential for effective clinical treatment, patient care, and research.