Abstract: Antibody-drug conjugates (ADCs) are considered next generation anti-cancer agents that improve therapeutic index. Traditional ADC using non-site-specific conjugation technique is a heterogenous mixture with different conjugation sites and carrying different numbers of drugs per antibody. The final ADC product contains different molecular structures with challenges to batch-to-batch consistency in product quality. The site-specific conjugation techniques have been developed to attach potent chemical drugs to defined sites of an antibody, which leads to homogeneous ADCs with better pharmacokinetics. In this article, we briefly summarize key components of ADCs, ADC characteristics, the development history for ADCs as well as the 9 approved ADCs, with focus on 5 major site-specific conjugation techniques leading to ADC products for clinical trials. Additionally, we summarize the clinical outcomes of ADCs based on site-specific conjugation techniques, with a prospect of their future application.