Respiratory diseases are pathological conditions that affect airways, hampering breathing and causing high mortality. In particular, asthma and allergic rhinitis (AR) are two of the most common airway diseases that affect millions of people and have a high prevalence in childhood and adulthood. Asthma is a heterogeneous chronic inflammatory disease characterized by wheezing, chest tightness, shortness of breath, and cough. AR occurs with rhinorrhea, nasal congestion, and sneezing. Indeed, these pathologies share common physiopathological mechanisms such as airway hyperresponsiveness and similar immunopathology such as tissue eosinophilia and T-helper type 2 inflammation. Moreover, AR can be an important risk factor for suffering asthma. Thus, early diagnosis and effective treatment are crucial to improving the health and quality of life of these patients. Classical drugs such as corticosteroids have been used; however, in the last decades, efforts to improve treatments have increased, focusing on biological agents and specific allergen immunotherapy development. Moreover, more precise diagnostic tools have been elaborated, besides classical methods (medical history, physical examination, and pulmonary function tests), such as basophil activation test, and specific cellular and molecular biomarkers (microRNAs, sputum/blood eosinophils, IgE serum, and periostin levels). Therefore, in this review, we compile all these important issues for managing asthma and AR.