The Eastern oyster Crassostrea virginica is a major aquaculture species for the United States. The sustainable development of Eastern oyster aquaculture depends on continued improvement of cultured stocks through advanced breeding technologies. The Eastern Oyster Breeding Consortium (EOBC) was formed to advance genetics and breeding of the E astern oyster. Members of EOBC have developed oyster strains with improved disease resistance through selective breeding and sequenced the Eastern oyster genome . T o facilitate efficient genotyping needed for genome-based analyses and breeding , the consortium developed two single-nucleotide polymorphism (SNP) arrays for the Eastern oyster: one with 566K SNPs and the other with 66K SNPs. The 566K array was developed based on resequencing data from 293 o ysters from Atlantic and Gulf populations . It contains 566 ,262 SNPs including 47K from protein coding genes and 33 SNPs from the mitochondrial genome . The 566K array was evaluated with 960 oysters yielding a sample passing rate of 87.89%, cluster calling rate of 98.89% and sample reproducibility of 99.24%. Of all SNPs on the array, 299,899 or 52.99 % were called at high resolution and recommended for downstream applications . The 66K array was developed using the best-performing 65,893 SNPs from the 566K array. Evaluation of the 66K array with 384 oysters yielded a sample passing rate of 94.00%, cluster calling rate of 99.34% and concordance of 99.81%. Of all SNPs on the array, 60,673 or 92.08% were called at high resolution and recommended for downstream applications , a big improvement in efficiency over the 566K array. Both arrays contained 756 probes from 13 oyster and human pathogens for their detection in genotyped oysters . The high-density 566K array is designed for marker screening and high-resolution genome-wide associ ation studies (GWAS), and the 66K array can provide efficient and cost-effective genotyping needed for routine GWAS and genomic selection . The development and implementation of these tools are expected to advance genomic research and accelerate genetic improvement of the Eastern oyster, by delineating genetic architecture of production traits, enabling genomic selection, increasing selection precision and l owering long-term breeding costs .