Antibody drug conjugate (ADC) is typically composed of a monoclonal antibody conjugated with a cytotoxic small molecule drug via a linker. It is an emerging and promising class of targeted cancer therapeutics, combining both the highly cytotoxic activity of chemical drugs and highly targeting ability and specificity of monoclonal antibody. Fourteen ADCs have been approved for marketing so far worldwide, and more than 140 ADC drug candidates have been investigated in clinical studies. Various ADC technologies have been well developed to manufacture these ADC drugs in commercial scale as well as clinical scale. In this review, we describe the molecular structure, mechanisms of action and development history of ADCs. We then provide an overview of the current landscape and recent advances in each key element of ADCs, including antibody, linker, payload and conjugation, and their advantages and disadvantages. Future directions in ADC development may encompass smaller sized forms of antibodies such as antibody fragments and nanobodies to improve the penetration and accumulation of ADCs in the solid tumors. Novel linkers are also being tested to enhance the stability in circulation systems and reduce off-target toxicities. Emerging payloads of new functional mechanisms are also explored in the construction of ADCs to overcome the drug resistance resulted from currently used payloads of marketed ADCs. Various site-specific conjugation technologies have been adopted to reduce the heterogeneity of drug-load species and optimize the pharmacokinetic properties of ADCs. This review article aims to enhance systemic understanding and careful considerations in designing an ADC drug with improved efficacy and safety.