The importance of genomic information in thyroid cancer has increased with the accumulation of whole genome analysis results. The fifth edition of the World Health Organization (WHO) classification of endocrine and neuroendocrine tumors (WHO fifth), released in 2022, has drastically revised the classification based on molecular genetic backgrounds. This has led to a systematic classification based on cellular origin and clinical risks. This chapter covers the latest pathological findings on thyroid cancers and their clinical implications. Key points include an overview of WHO fifth, genetic alterations in thyroid tumors, and the differential diagnosis of “low-risk” and “high-risk” tumors. Finally, a three-step algorithm for the standardization of pathological diagnosis is proposed. Understanding genetic alterations has become essential not only for the pathological diagnosis of thyroid tumors but also for selecting appropriate treatment options.