Allergic diseases, including asthma, allergic rhinitis, atopic dermatitis, and food allergies, are driven by dysregulated immune responses, often involving IgE-mediated mast cell and basophil activation, Th2 inflammation, and epithelial dysfunction. While environmental factors are well-known contributors, the genetic components underpinning these conditions are increasingly understood. Traditionally viewed as polygenic multifactorial disorders, allergic diseases can also be caused by single-gene defects affecting the immune system and skin epithelial barrier, leading to profoundly dysregulated allergic responses. These monogenic allergic disorders are collectively referred to as primary atopic disorders or PADs. To date, over 48 single-gene defects have been established to cause PADs. This review highlights (i) the significance of PADs, (ii) the biological pathways involved in the pathogenesis of PADs, (iii) clinical strategies to differentiate PADs from their much more common polygenic counterparts, and (iv) diagnostic strategies for PADs.