This issue of Immunology and Allergy Clinics, guest edited by Mariana C. Castells, MD, is devoted to Mastocytosis. Articles in this issue include: New Insight into Clonal Mast Cell Disorders Including Mastocytosis; Cutaneous Mastocytosis in Adults and Children: New Classification and Prognostic Factors; Hymenoptera Anaphylaxis as a Clonal Mast Cell Disorder; Non Clonal Mast Cell Activation Syndrome:  A growing body of evidence; Familial Tryptasemia Syndromes: Genotyping and Symptoms; POTS and EDS: Entities Associated to Mast Cell Activation; KIT Mutations: New Insight and Diagnostic Value; Patients’ Perceptions in Mast Cell Activation Disorders; Mast Cell Mediators of Significance in Clinical Practice; Tyrosine Kinase Inhibition in Mast Cell Activation Disorders; Gastrointestinal Involvement in Mast Cell Activation Disorders; Bone Marrow Expression of Mast Cell Disorders; Genomics and Proteonomics in Clonal Mast Cell Disorders; and Pediatric Expression of Mast Cell Activation Disorders.