Curriculum-based measurement (CBM) has conventionally included accuracy criteria with recommended fluency thresholds for instructional decision-making. Some scholars have argued for the use of accuracy to directly determine instructional need (e.g., Szadokierski et al., 2017). However, accuracy and fluency have not been directly examined to determine their separate and joint value for decision-making in CBM prior to this study. Instead, there was an assumption that instruction that emphasized accurate responding should be monitored with accuracy data, which evolved into the use of complementing CBM fluency scores with accuracy or using timed assessment to compute percent of responses correct and using accuracy criteria to determine instructional need. The purpose of this article was to examine fluency and accuracy as related but distinct metrics with psychometric properties and associated benefits and limits. Findings suggest that the redundancy between accuracy and fluency causes them to perform comparably overall, but that: (1) fluency is superior to accuracy when accuracy is computed on a timed sample of performance; (2) timed accuracy adds no benefit relative to fluency alone; and (3) accuracy when collected under timed assessment conditions has substantial psychometric limitations that make it unsuitable for the formative instructional decisions which are commonly made using CBM data. The conventional inclusion of accuracy criteria in tandem with fluency criteria for instructional decision-making in CBM should be reconsidered as there may be no added predictive value, but rather additional opportunity for error due to the problems associated with unfixed trials in timed assessment.