Several baseball and softball physical properties relate directly or indirectly to the risk of head injury, namely size, weight, hardness, and liveliness. Rule book specifications for the official game balls in baseball and Softball specify the size and weight. Ball liveliness is sometimes specified. This study has evaluated the effects of ball hardness and liveliness on the risk of head injury in impact studies with humanoid head models, rigid head forms, and actual cadaver impact tests. The Severity Index (SI) and Head Injury Criteria (HIC) were both measured over a wide range of ball hardness and impact speeds. The risk of head injury was determined using the Prasad-Mertz risk curves. The results reveal a very strong relationship between ball hardness and head injury risk, ranging from 80% risk at 28.6 m/s (60 mph) for popular hard baseballs and softballs, down to 1% risk for softer balls now being used in some youth league programs. The cadaver impact tests confirmed that the humanoid head model and Severity Index injury criteria are well suited for the prediction of head injury risk for highly focal (concentrated in small area) blows from baseballs and softballs to the side of the unprotected head. Field test injury statistics show that the use of softer type baseballs in youth league play can reduce the incidence of ball impact injuries by about 70%.