Progesterone (P4), a steroid hormone, is widely recognized for its vital function in maintaining pregnancy across various animal species. Its functions extend beyond pregnancy management, encompassing the determination of pregnancy, optimal reproductive timing, anticipation of parturition, scheduling of elective cesarean sections, and identification of conditions like growth hormone disorders, insulin-dependent diabetes, and infertility. Its versatility extends to involvement in bone marrow trophication, stress response assessment, and neuroprotection following traumatic brain injuries or fetal hypoxia. Although the aforementioned are mostly directed toward females, it is important to note that progesterone is also used clinically in males. Monitoring blood progesterone levels in animals is essential in both healthy and pathological states, as emphasized in this chapter.