Inclusion Criteria:  General:  Between 6-17 years of age Doctor diagnosis of asthma  In addition to the general criteria, a case with uncontrolled asthma (case group A) should meet:  Asthma treatment Global INitiative of Asthma (GINA) step 3 or higher  ≥ 1of the following criteria (based on the guideline 'Childhood Asthma' of the section paediatric pulmonology of the Dutch Society of Paediatricians, 2007):  Frequent exacerbations requiring OCS use (≥1 in the past year) and/or Severe exacerbations requiring hospitalization or ER visits in the past year and/or ACQ/ACT scores indicating uncontrolled asthma  In addition to the general criteria, a case with an acute exacerbation (case group B) should meet:  Asthma treatment Global INitiative of Asthma (GINA) step 2 or higher Current severe asthma exacerbation requiring hospitalization  In addition to the general inclusion criteria, a control (controlled asthma) should meet the following criteria:  Asthma treatment Global INitiative of Asthma (GINA) step 3 or higher Lack of severe exacerbations requiring OCS use or hospitalizations or ER visits in the past year ACQ/ACT scores indicating well controlled asthma or few reported asthma symptoms during asthma checks in the past year.  Exclusion Criteria:  Recent use of a course of antibiotics (< 1 month). This will affect the microbiome analyses.  A patient can be re-screened 1 month after the use of the antibiotic treatment has finished.