The pulmonary dosing route has been advocated as an attractive alternative to injection and oral administration for the systemic delivery of therapeutic peptides and proteins. The lung possesses many favorable physiological characteristics for systemic absorption of inhaled peptides/proteins, so inhalable formulation systems of these drugs have generated considerable interest as a valid and non-invasive dosing approach. A major obstacle to the widespread use of inhalation therapies for many peptides/proteins is the limited bioavailability and thereby insufficient therapeutic outcomes because of biopharmaceutical challenges such as rapid pulmonary clearance, limited pulmonary deposition, delayed dissolution in lung environment, poor membrane permeability, and low metabolic stability. A better understanding of the biopharmaceutical properties of inhaled peptides/proteins would be indispensable to overcome these drawbacks with the aid of strategic drug delivery systems and chemical synthesis of new derivatives based on structure-activity relationship information, possibly leading to improved therapeutic potential of pharmaceutical products. The present paper reviews biopharmaceutical properties of inhaled peptides/proteins, with a focus on the pharmacokinetic fate of inhaled peptides/proteins and critical determinants of therapeutic potential. The emphasis in this mini-review will also be on viable formulation approaches for breakthroughs beyond the biopharmaceutical limitations of inhalation therapy with peptides/proteins.