<p>Diazotization coupling reactions are highly effective for spectrophotometric analysis and are particularly useful for the sensitive detection of pharmaceutical compounds and environmental contaminants, such as nitrate and nitrite. This review examines primary literature published since 2010, with a focus on structural modifications, reaction mechanisms, and practical analytical applications of azo-based spectrophotometric techniques. Pharmaceutical analytes are categorized by functional class, including aromatic amines, nitro compounds, phenols, hydrazine derivatives, and aliphatic amines. For each class, we describe the relevant diazotization pathways and related processes, such as hydrolysis and chemical reduction. Additionally, the review examines environmental applications, with a focus on methods for quantifying nitrate and nitrite in various matrices, including water. When applicable, we discuss sample preparation strategies, such as solid-phase extraction and related preconcentration techniques, which enhance sensitivity and selectivity. We also highlight common limitations, such as diazonium instability, matrix interferences, and nonspecific azo formation, and present practical mitigation strategies, including pH and temperature control, careful reagent selection, and coupling with chromatographic separation. Overall, diazotization coupling remains a cost-effective and versatile approach, combining simplicity with high sensitivity. It is suitable for both routine quality control and targeted environmental monitoring.</p>