Primary Aldosteronism (PA) is caused by autonomous overproduction of aldosterone. Aldosterone is necessary for fluid and ion homeostasis. Aberrant overproduction leads to hypertension and cardiovascular damage. With a prevalence of over 5% in the worlds’ hypertensive community, and with over a billion people worldwide having high blood pressure, PA represents a major contributor to health care costs and morbidity. Importantly, 30% of PA patients have a unilateral dominant secretion, an aldosterone producing adenoma (APA), making it possible to cure a substantial portion of patients with surgery. Unfortunately, there is a large underdiagnosis of PA, leading to delayed diagnosis in many patients, worsening their outcome after surgery. A need for better screening techniques, raised awareness and treatment options for PA is warranted. Since 2011, the genetic understanding of APAs has revolutionized. Somatic mutations turning on an autonomous aldosterone production has been observed in up to 80% of tumors. In this thesis we have investigated the genetic landscape and phenotypes of APAs. By international collaborations we have collected one of the largest cohorts of APAs ever sequenced. We have confirmed and extended the understanding of KCNJ5 mutations, its associated phenotype and the specificity for APAs. We have confirmed a high rate of mutations in ATP1A1, ATP2B3 and CACNA1D, and noted distinct clinical and molecular phenotypes in these tumors. We describe a marker of Zona Glomerulosa cells, perhaps important for the normal regulation and function of these cells. We observe somatic mutations in CTNNB1, occurring in a mutually exclusive manner to the other mutations. Using in situ sequencing, we note genetic heterogeneity in APAs with KCNJ5 mutations. Finally, we evaluate intratumoral aldosterone measurement on a large cohort of tumors, validating a high specificity for APAs. Noting also a difference in the level of intratumoral aldosterone between APAs and a possible association with genotype. Remarkably, we also note a robust correlation between the intracellular concentrations and plasma-aldosterone. We hope that with gained knowledge of the genetic background, the understanding of both pathologic and normal states of the adrenals will increase, and hopefully benefit patients in the future.