In the past decade, an increased amount of clinically-oriented research involving immunotoxins has been published. Immunotoxins are a group of artificially-made cytotoxic molecules targeting cancer cells. These molecules composed of a targeting moiety, such as a ligand or an antibody, linked to toxin moiety, which is a toxin with either truncated or deleted cell-binding domain that prevents it from binding to normal cells. Immunotoxins can be divided into two categories: chemically conjugated immunotoxins and recombinant ones. The immunotoxins of the first category have shown limited efficacy in clinical trials in patients with hematologic malignancies and solid tumors. Within the last few years, single-chain immunotoxins provide enhanced therapeutic efficacy over conjugated forms and result in improved antitumor activity. In this review, we briefly illustrate the design of the immunotoxins and their applications in clinical trials.