Ketosis - negative energy balance leading to excessive ketone body production in early lactation dairy cows - is one of the most prevalent metabolic diseases in high-producing dairy herds, affecting welfare through illness, pain, and secondary disease risk. Clinical ketosis causes anorexia, reduced milk production, and neurological signs (nervous ketosis). Subclinical ketosis (elevated BHBA without clinical signs) affects 30-50% of cows in the first 2 weeks of lactation and significantly increases risk of displaced abomasum, metritis, lameness, and reproductive failure. Detection using on-farm cowside BHBA testing (Precision Xtra meters) enables systematic herd-level monitoring and early intervention. Propylene glycol drenching and IV dextrose treat clinical cases; monensin supplementation in dry period reduces incidence. Welfare-positive prevention focuses on transition cow management: avoiding over-conditioning (BCS>3.5 at calving), providing adequate rumen fill, and ensuring smooth dry period to lactation transition.