Primary adrenal insufficiency (PAI) is a critical condition that requires prompt diagnosis and initiation of treatment. Diagnosis can be challenging due to various underlying causes, including defects in adrenal gland development, resistance to adrenocorticotropic hormone, autoimmune causes, and metabolic problems. A specific diagnosis is essential for developing a treatment plan and identifying other possible accompanying pathologies. Biochemical studies, genetic analyses, and imaging techniques are helpful in establishing a specific diagnosis. This evidence-based guideline includes the specific diagnoses that cause PAI and their clinical and genetic features. It also provides evidence-based steps to follow when making a diagnosis.