Two problems exist in current definitions of reasonable doubt. First, although every jurisdiction requires by statute that a defendant's guilt be proven beyond a reasonable doubt, courts and legislatures have defined this concept differently. Second, the language used to define reasonable doubt, whether found in statute or case law, is derived primarily from the language of a case that is over 100 years old. The result is that jury instructions defining reasonable doubt are often couched in archaic and unclear language. Further, research indicates that juror understanding of instructions is extremely low and that juror comprehension of instructions increases dramatically when such instructions are written with the lay person in mind. The U.S. Supreme Court has demonstrated its reluctance to prescribe a definition of reasonable doubt, and the message of recent decisions is that States must re-examine their definitions of reasonable doubt to ensure the language conveys the degree of certainty mandated by due process. The authors conclude the U.S. Supreme Court needs to issue clearer guidance on the question of reasonable doubt for use by trial courts. 134 references, 4 notes, and 5 tables