Parasitoid insects are veritable serial killers, and they play a major role in the regulation of insect populations. Scientific research on the lines of a criminal investigation reveals how these killers behave and what remarkable capabilities they have. In basic research, parasitoids make excellent models for behavioral ecology studies. They are also used in applied research, notably for biological control, where they can serve as precious auxiliaries for controlling crop pests. Parasitoids still have many secrets to reveal, and new research avenues are opening up.