Monoclonal antibodies (mAbs) and derivatives are currently the fastest growing class of therapeutic molecules. More than 30 G-type immunoglobulins (IgG) and related agents have been approved over the past 25 years mainly for cancers and inflammatory diseases. In oncology, mAbs are often combined with cytotoxic drugs to enhance their therapeutic efficacy. Alternatively, small anti-neoplastic molecules can be chemically conjugated to mAbs, used both as carriers (increased half-life) and as targeting agents (selectivity). Potential benefits of antibody-drug conjugates (ADCs), strategies, and development challenges are discussed in this review. Several examples of ADCs are presented with emphasis on three major molecules currently in late clinical development as well as next generation thio-mAbs conjugates with improved therapeutic index.