Corticosteroids remain among the most important anti-inflammatory drugs in respiratory medicine, and their benefit-risk profile differs substantially between asthma and chronic obstructive pulmonary disease (COPD). In asthma, inhaled corticosteroids (ICS) are the foundational controller therapy because they reduce type 2 airway inflammation and symptoms, improve lung function, reduce exacerbations, and are associated with lower asthma mortality. Moreover, the use of ICS-beta2 agonists as both controller and reliever therapy has further reframed asthma treatment around rapid delivery of anti-inflammatory therapy. Systemic corticosteroids are effective for acute asthma exacerbations and, in selected patients with severe asthma, may be required as maintenance therapy. Yet, cumulative oral corticosteroid exposure is associated with substantial acute and chronic toxicity. In COPD, corticosteroid pharmacology is more selective. ICS are not universally used as anti-inflammatory therapy; they are most useful in exacerbation-prone patients, particularly those with higher blood eosinophil counts or concomitant asthma and are generally used in combination with long-acting bronchodilators. In COPD, ICS can reduce exacerbations in selected patients but can increase pneumonia risk. Systemic corticosteroids improve outcomes during acute COPD exacerbations, with short courses generally preferred, but chronic oral corticosteroids are not recommended for stable COPD because toxicity exceeds benefits. This review examines the clinical pharmacology, therapeutic role, and toxicology of inhaled and systemic corticosteroids in asthma and COPD separately, emphasizing precision use, cumulative exposure reduction, and monitoring strategies.