A scientific literature review and consensus of expert opinion used the welfare definitions provided by the Farm Animal Welfare Council (FAWC) Five Freedoms as the framework for selecting a set of animal-based indicators that were sensitive to the current on-farm welfare issues of young lambs (aged = 0.70, Sp >= 0.98), stimulation (kappa = 0.57, Se = 0.30 to 0.77, Sp >= 0.98), shivering (kappa = 0.55, Se = 037 to 0.85, Sp >= 0.99), standing ability (0.54, Se >= 0.80, Sp >= 0.99), posture (kappa = 0.45, Se 0.56, Sp = 0.99), abdominal fill (kappa = 0.44, Se = 0.39 to 0.98, Sp = 0.99), body condition (kappa = 0.72, Se >= 0.38 to 0.90, Sp = 0.99), lameness (kappa = 0.68, Se > 0.73, Sp = 1.00), and eye condition (kappa = 0.72, Se >= 0.86, Sp = 0.99). LCA predicted that randomly selected observers had Se > 0.77 (acceptable), and Sp 0.98 (high) for assessments of demeanour, lameness, abdominal fill posture, body condition and eye condition. The diagnostic performance of some indicators was influenced by the composition of the study population, and it would be useful to test the indicators on lambs with a greater level of outcomes associated with poor welfare. The findings presented in this paper could be applied in the selection of valid, reliable and feasible indicators used for the purposes of on-farm assessments of lamb welfare.