The article discusses the importance of replication in genotype-phenotype association studies, emphasizing the need to validate initial findings to ensure their credibility. With the completion of the human genome and the mapping of SNPs, genome-wide association studies (GWAS) have become prevalent, but many reported associations have not been consistently replicated. This highlights the challenge of distinguishing true associations from false positives. Replication studies are essential to confirm findings, whether from candidate-gene or genome-wide approaches. However, there is no consensus on what constitutes a valid finding for replication, how to design adequate replication studies, or what constitutes a successful replication. The NCI-NHGRI working group on replication in association studies convened experts from various fields to address these issues. They outlined key considerations for replication studies, including study design, data quality, and the importance of comparing results across different populations. They also emphasized the need for detailed reporting in publications to facilitate independent verification of findings. Challenges include small sample sizes, poor study design, and differences in genetic variation across populations. Additionally, the use of different variants in follow-up studies can complicate replication efforts. The group proposed best practices for designing, conducting, and publishing replication studies, including the use of standardized criteria for defining phenotypes, ensuring data availability, and maintaining high-quality genotyping and quality control. They also highlighted the importance of replication in different populations and the role of family-based studies in reducing population stratification. Negative studies, which are often overlooked, are also crucial for validating findings and should be published to help distinguish true positives from false positives. The article concludes that replication is essential for establishing the validity of genotype-phenotype associations, and that future studies should focus on improving study design, data sharing, and the interpretation of results. The importance of replication in the context of genome-wide association studies is underscored, with the need for continued methodological improvements to enhance the reliability of genetic findings.The article discusses the importance of replication in genotype-phenotype association studies, emphasizing the need to validate initial findings to ensure their credibility. With the completion of the human genome and the mapping of SNPs, genome-wide association studies (GWAS) have become prevalent, but many reported associations have not been consistently replicated. This highlights the challenge of distinguishing true associations from false positives. Replication studies are essential to confirm findings, whether from candidate-gene or genome-wide approaches. However, there is no consensus on what constitutes a valid finding for replication, how to design adequate replication studies, or what constitutes a successful replication. The NCI-NHGRI working group on replication in association studies convened experts from various fields to address these issues. They outlined key considerations for replication studies, including study design, data quality, and the importance of comparing results across different populations. They also emphasized the need for detailed reporting in publications to facilitate independent verification of findings. Challenges include small sample sizes, poor study design, and differences in genetic variation across populations. Additionally, the use of different variants in follow-up studies can complicate replication efforts. The group proposed best practices for designing, conducting, and publishing replication studies, including the use of standardized criteria for defining phenotypes, ensuring data availability, and maintaining high-quality genotyping and quality control. They also highlighted the importance of replication in different populations and the role of family-based studies in reducing population stratification. Negative studies, which are often overlooked, are also crucial for validating findings and should be published to help distinguish true positives from false positives. The article concludes that replication is essential for establishing the validity of genotype-phenotype associations, and that future studies should focus on improving study design, data sharing, and the interpretation of results. The importance of replication in the context of genome-wide association studies is underscored, with the need for continued methodological improvements to enhance the reliability of genetic findings.