This review discusses the structure, viscoelastic properties, and composition of mucus in chronic respiratory diseases (CRDs), including asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF). Mucus, a viscoelastic gel, plays a critical role in airway defense through mucociliary clearance. In CRDs, mucus overproduction and altered composition disrupt this defense mechanism, leading to airway damage and a cycle of obstruction and infection. The review highlights key features of normal and pathological mucus, such as mucin content, structure, micro/macro-rheology, pH, and osmotic pressure. It also discusses the potential of sputum biomarkers, including mucins, inflammatory proteins, and metabolites, as indicators of disease severity and therapeutic response. The review emphasizes the need for further research to validate mucus structure and viscoelasticity as biomarkers for CRD pathologies. It also explores the role of mucus in the pathophysiology of CRDs, including the involvement of mucins, goblet cells, and submucosal glands. The review discusses the mechanisms underlying mucus hypersecretion in CRDs, including the role of CFTR dysfunction, inflammatory mediators, and other signaling pathways. The review also examines the structural and functional characteristics of mucus in CRDs, including the viscoelastic properties of mucus and its role in airway defense. The review concludes that mucus structure and viscoelasticity are important factors in the pathophysiology of CRDs and may serve as useful biomarkers for disease monitoring and treatment.This review discusses the structure, viscoelastic properties, and composition of mucus in chronic respiratory diseases (CRDs), including asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF). Mucus, a viscoelastic gel, plays a critical role in airway defense through mucociliary clearance. In CRDs, mucus overproduction and altered composition disrupt this defense mechanism, leading to airway damage and a cycle of obstruction and infection. The review highlights key features of normal and pathological mucus, such as mucin content, structure, micro/macro-rheology, pH, and osmotic pressure. It also discusses the potential of sputum biomarkers, including mucins, inflammatory proteins, and metabolites, as indicators of disease severity and therapeutic response. The review emphasizes the need for further research to validate mucus structure and viscoelasticity as biomarkers for CRD pathologies. It also explores the role of mucus in the pathophysiology of CRDs, including the involvement of mucins, goblet cells, and submucosal glands. The review discusses the mechanisms underlying mucus hypersecretion in CRDs, including the role of CFTR dysfunction, inflammatory mediators, and other signaling pathways. The review also examines the structural and functional characteristics of mucus in CRDs, including the viscoelastic properties of mucus and its role in airway defense. The review concludes that mucus structure and viscoelasticity are important factors in the pathophysiology of CRDs and may serve as useful biomarkers for disease monitoring and treatment.