PURPOSE OF REVIEW: As an ex-vivo test of allergic effector cell activation, basophil activation testing (BAT) to allergen enables quantification of the in-vivo IgE-mediated allergic response. BAT thus holds promise in the diagnosis and monitoring of peanut and tree nut allergies. Recent systematic analyses and expert recommendations support a role for BAT in the diagnosis of peanut and tree nut allergy. RECENT FINDINGS: Diagnostic cut-offs for BAT in peanut and tree nut allergy have been identified. Consistently, BAT can discriminate with high sensitivity and specificity between allergy and tolerance when measured against oral food challenges. Furthermore, the utilization of BAT has can increase the sensitivity and specificity of peanut allergy and tree nut allergy diagnosis, both alone and in conjunction with specific IgE testing and skin prick testing. BAT is a promising tool in the diagnosis of peanut and tree nut allergy.