The growth rates of Malaysia and Thailand are generally lower than those of Northeastern Asian countries such as South Korea and China when compared at the same level of per capita income. This difference could be caused by differences in the degree of export/industrial upgrading. In the first half of this paper, we construct an export sophistication index and decompose it into within-and between-production-stage factors. We find that both Northeast and Southeast Asian countries have successfully upgraded their export structure within each production stage during the period 1990-2015. However, after 2000, the Southeast Asian countries have failed to upgrade their export structure to a more sophisticated production stage, such as capital goods and parts/components. In the second half of this paper, we analyze trade in value added using international input-output data to investigate the value added content of high-tech exports. We find a significant change in the share of high-tech exports when considering the share of domestic content in exports. Furthermore, Southeast Asian countries have notably high vertical specialization shares (i.e., the share of foreign content in exports) because they heavily depend on intermediate inputs supplied by Northeast Asian countries. Moreover, the flying geese pattern of industrial development, which reflects export structure upgrading from downstream to upstream industries, is not observed in Southeast Asian countries.