In this review, we address the optical signatures of defects in two-dimensional transition metal dichalcogenides (2D TMDs), whether they occur unintentionally during growth or are deliberately introduced post-growth. We detail their primary responses as probed by photoluminescence (PL), magneto-PL, Raman, tip-enhanced PL and Raman, and nonlinear spectroscopies. Defects significantly impact the electronic, vibrational, magneto-optical, and nonlinear properties of TMDs, influencing outcomes based on application needs. This comprehensive overview highlights the distinctive optical fingerprints of various defects, providing guidance for their identification and characterization. Additionally, we discuss new optical phenomena induced by defects in TMD monolayers and future challenges in defect engineering for these materials. The insights from this review underscore the potential of TMDs for technological applications, with advancements in spectroscopy and defect engineering driving future innovations and enhancing our understanding of these materials.