Inclusion Criteria:  Raw score of 14 or more on OAB-q SF (adjusted score of 20) Postmenopausal defined by not having a period for 12 months or bilateral oophorectomy a least 12 months status post procedure, or women with prior hysterectomy and preserved ovaries over age 55 years old or having a documented follicle stimulating hormone level greater than 40mIU/mL. Ability to speak and read English  Exclusion Criteria:  Contraindications to Vaginal Estrogen or Mirabegron Post void residual >200mL or >1/3 patient's total bladder volume Currently on or previously taking anticholinergic or beta- 3 agonists within the past 1 month Symptomatic pelvic organ prolapse greater than Stage 2, contraindication to vaginal estrogen therapy Undiagnosed postmenopausal vaginal bleeding within the past 12 months Current diagnosis of Multiple sclerosis, Parkinson's Disease, prior spinal cord injury, Spina Bifida, prior pelvic irradiation, Interstitial Cystitis, recurrent urinary tract infection.