Research during the early years of dye synthesis produced compounds such as methylene blue and acriflavine which were used as biological stains. The selectivity of such compounds for “non-economic” cells such as pathogenic bacteria or tumour cells gave rise both to the principle of selective toxicity, and to the development of modern drugs, for example in the field of tropical medicine.The use of dyes in therapy is again gaining credence today, given the efficacy of light-activated drugs based on dye molecules against drug-resistant organisms such as MRSA. In addition, older drugs developed from dye chromophores may again be of use in the clinic due to the continuing rise of the “Superbugs”.