Epigenetic modifications, including DNA methylation and histone modifications, 17 are reprogrammed considerably following fertilization during mammalian early embryonic 18 development. Incomplete epigenetic reprogramming is a major factor leading to poor 19 developmental outcome in embryos generated by assisted reproductive technologies, such as 20 somatic cell nuclear transfer. However, the role of histone modifications in preimplantation 21 development is poorly understood. Here, we show that co-knockdown (cKD) of Hdac1 and 2 22 (but not individually) resulted in developmental failure during the morula to blastocyst 23 transition. This outcome was also confirmed with the use of small-molecule Hdac1/2-specific 24 inhibitor FK228. We observed reduced cell proliferation and increased incidence of apoptosis 25 in cKD embryos, which were likely caused by increased acetylation of Trp53. Importantly, both 26 RNA-seq and immunostaining analysis revealed a failure of lineage specification to generate 27 trophectoderm and pluripotent cells. Among many gene expression changes, a substantial 28 decrease of Cdx2 may be partly accounted for by the aberrant Hippo pathway occurring in cKD 29 embryos. In addition, we observed an increase in global DNA methylation, consistent with 30 increased DNA methyltransferases and Uhrf1. Interestingly, deficiency of Rbbp4 and 7 (both 31 are core components of several Hdac1/2-containing epigenetic complexes) results in similar 32 phenotypes as those of cKD embryos. Overall, Hdac1 and 2 play redundant functions required 33 for lineage specification, cell viability and accurate global DNA methylation, each contributing 34 to critical developmental programs safeguarding a successful preimplantation development. 35 36 Significance 37 Substantial changes to epigenetic modifications occur during preimplantation development and 38 can be detrimental when reprogrammed incompletely. However, little is known about the role 39 of histone modifications in early development. Co-knockdown of Hdac1 and 2, but not 40 individually, resulted in developmental arrest during morula to blastocyst transition, which was 41 accompanied by reduced cell number per embryo and increased incidence of apoptosis. 42 Additionally, we observed a failure of first lineage specification to generate trophectoderm and 43 pluripotent cells, which were associated with reduced expression of key lineage-specific genes 44 and aberrant Hippo pathway. Moreover, an increase in global DNA methylation was found with 45 upregulated Dnmts and Uhrf1. Thus, Hdac1 and 2 play overlapping roles in lineage 46 development, apoptosis, and global methylation during preimplantation development.47 48