Drug delivery systems are generally believed to comprise drugs and excipients. A peptide-drug conjugate is a single molecule that can simultaneously play multiple roles in a drug delivery system, such as in vivo drug distribution, targeted release, and bioactivity functions. This molecule can be regarded as an integrated drug delivery system, so it is called a molecular drug delivery system. In the context of cancer therapy, a peptide-drug conjugate comprises a tumor-targeting peptide, a payload, and a linker. Tumor-targeting peptides specifically identify membrane receptors on tumor cells, improve drug-targeted therapeutic effects, and reduce toxic and side effects. Payloads with bioactive functions connect to tumor-targeting peptides through linkers. In this review, we explored ongoing clinical work on peptide-drug conjugates targeting various receptors. We discuss the binding mechanisms of tumor-targeting peptides and related receptors, as well as the limiting factors for peptide-drug conjugate-based molecular drug delivery systems. Towards enhanced targeting on tumorsDespite the rapid development of anticancer drugs in the past few decades, cancer is one of the leading causes of death in the world [1]. Due to the lack of an effective means to distinguish tumor cells from normal cells [2], the accumulation efficiency of anticancer drugs in tumor cells is low and the toxic side effects on normal tissues are high. In the treatment process, drug resistance problems are caused by tumor heterogeneity [3], drug inactivation, transport, and metabolism, changes in drug targets, and tumor microenvironment dysfunction [4]. Although advanced drug delivery methods can partially solve problems of targeting, toxicity, and drug resistance [5], difficult industrialization, complicated process control, and huge costs remain insurmountable obstacles.Peptide-drug conjugates are emerging molecular drug delivery system (see Glossary) that achieve precise drug delivery and tumor-targeted release at the molecular level [6]. Current drug delivery systems, such as nano-drug delivery systems, use various complex excipients to assemble nanoscale particles that encapsulate drugs to be passively or actively delivered to target organs. The anticancer efficacy of nano-drug delivery systems is improved by an enhanced permeability retention effect to increase drug accumulation in tumors and the toxicity of nanodrug delivery systems is reduced by the long systemic circulation to decrease drug accumulation in normal organs [7]. However, a reappraisal of nano-drug delivery system design is suggested to improve their clinical efficacy and safety in cancer patients [8].By contrast, molecular drug delivery systems use a single peptide-drug conjugate molecule to achieve in vivo drug delivery, targeted drug release, and bioactivity, which can maximize the targeting effect and reduce drug resistance. Peptide-drug conjugates, which were first reported in 1972 by Freer et al.[9] (Box 1), are similar to the tethering of ligand-targeted drugs [10],