Poorly differentiated thyroid carcinoma (PDTC) and anaplastic thyroid carcinoma (ATC) are aggressive thyroid tumors associated with a high mortality rate of 38-57 % and almost 100 % respectively. Several recent studies utilizing next generation sequencing techniques have shed lights on the molecular pathogenesis of these tumors, providing evidence to support a stepwise tumoral progression from well-differentiated to poorly differentiated, and finally to anaplastic thyroid carcinomas. While BRAF (V600E) and RAS mutations remain the main drivers in aggressive thyroid carcinoma, PDTC and ATC gains additional mutations, e.g., TERT promoter mutation, TP53 mutation, as well as frequent alterations in PIK3CA-PTEN-AKT-mTOR pathway, SWI-SNF complex, histomethyltransferases, and mismatch repair genes. RAS-mutated PDTCs are commonly associated with a histologic phenotype defined by Turin proposal, high frequency of distant metastasis, high thyroid differentiation score, and a RAS-like gene expression profile, whereas BRAF-mutated PDTCs are usually defined solely by the Memorial Sloan Kettering Cancer Center (MSKCC) criteria with a propensity for nodal metastasis and are less differentiated with a BRAF-like expression signature. Such demarcation is largely lost in ATC which is characterized by genomic complexity, heavy mutation burden, and profound undifferentiation. Additionally, several molecular events, e.g., EIF1AX mutation, mutation burden, and chromosome 1q gain in PDTCs, as well as EIF1AX mutation, chromosome 13q loss, and 20q gains in ATCs, may serve as adverse prognostic markers predicting poor clinical outcome.