Mucus is a complex biological material that lubricates and protects various mucosal surfaces in the human body. Its rheological properties, which vary across different length scales, are crucial for its functions as a physical barrier and a lubricant. At the macroscale, mucus behaves as a non-Newtonian gel, while at the nanoscale, it acts as a low-viscosity fluid. Advances in rheological characterization have contributed to a better understanding of mucus physiology, disease pathology, and the development of drug delivery systems. This article reviews the biochemical regulation of mucus rheology, the macro- and microrheology of human and animal mucus, and the importance of understanding these properties for advancing drug and gene delivery. The rheological techniques used to characterize mucus across different length scales are also discussed. The distinction between macro- and microrheology is highlighted, with microrheology focusing on the viscoelastic properties encountered by micro- and nanoscale entities. The biochemical factors governing mucus rheology, including mucins, DNA, lipids, salts, proteins, and cells, are detailed. The importance of mucus microrheology in drug and gene delivery is emphasized, as it helps overcome the mucus barrier. The macrorheology of human mucus, including respiratory, gastrointestinal, and cervical mucus, is reviewed, along with its implications for lung function, reproductive biology, and disease transmission. Animal mucus models, particularly those from dogs, pigs, and rats, are also discussed. Finally, classical macro-rheological techniques such as cone & plate rheometry and other biophysical methods are described for characterizing mucus rheology.Mucus is a complex biological material that lubricates and protects various mucosal surfaces in the human body. Its rheological properties, which vary across different length scales, are crucial for its functions as a physical barrier and a lubricant. At the macroscale, mucus behaves as a non-Newtonian gel, while at the nanoscale, it acts as a low-viscosity fluid. Advances in rheological characterization have contributed to a better understanding of mucus physiology, disease pathology, and the development of drug delivery systems. This article reviews the biochemical regulation of mucus rheology, the macro- and microrheology of human and animal mucus, and the importance of understanding these properties for advancing drug and gene delivery. The rheological techniques used to characterize mucus across different length scales are also discussed. The distinction between macro- and microrheology is highlighted, with microrheology focusing on the viscoelastic properties encountered by micro- and nanoscale entities. The biochemical factors governing mucus rheology, including mucins, DNA, lipids, salts, proteins, and cells, are detailed. The importance of mucus microrheology in drug and gene delivery is emphasized, as it helps overcome the mucus barrier. The macrorheology of human mucus, including respiratory, gastrointestinal, and cervical mucus, is reviewed, along with its implications for lung function, reproductive biology, and disease transmission. Animal mucus models, particularly those from dogs, pigs, and rats, are also discussed. Finally, classical macro-rheological techniques such as cone & plate rheometry and other biophysical methods are described for characterizing mucus rheology.