Several recent studies that employed DNA fingerprinting to identify crop varieties on farmers' fields revealed a significant mismatch between perceived and actual adoption. Varietal misidentification may undermine seed market functioning, especially if low-quality seeds of age-old varieties are distributed through the same channels as seeds of recently-released improved varieties. Farmers' misleading notions on the characteristics of the cultivated variety may lead to inadequate agronomic management practices, affecting crop yield and farm profit. From a monitoring and impact evaluation perspective, incorrect variety classification may distort the composition of treatment and control groups, resulting in biased estimates. By systematically reviewing applications of DNA fingerprinting for varietal identification in farmers' fields, we compiled information from 23 published studies to generate an overview of the magnitude of farmer variety misclassification. We discuss the implications of crop variety misclassification for farm management and revenue, the correlates of variety misidentification, and how measurement error affects adoption and impact estimates. The review aims at helping researchers and policymakers strategize to more effectively assess the functioning and effectiveness of seed diffusion systems to deliver modern seeds to smallholders. It would also sensitize adoption and impact researchers on the various risks of varietal misclassification.