Cancer is currently one of the leading causes of death in the world, and different classes of antitumor drugs are used for the treatment. Traditional cytostatics act not only on tumor cells but also on some healthy cells and thus show a toxic effect. Despite the development of new, targeted drugs, the problem of ineffectiveness and toxicity of therapy is still present. Due to large differences in efficacy and toxicity both in different patients and in each individual patient, it is desirable to introduce therapeutic drug monitoring (TDM). Already in early clinical trials, the therapeutic and toxic doses of a drug is determined by pharmacokinetic and pharmacodynamic studies. However, depending on the stage and type of the disease, as well as the characteristics of the patient, it is possible that the effective doses will be significantly lower than the maximum tolerated dose, or that even the doses above which significant toxicity occurs will not lead to an adequate effect. Despite the fact that in most cases the determination of the concentration of drugs in the body ends with their registration, for some drugs patients could benefit from therapeutic monitoring. Drugs that are good candidates for TDM must meet a number of pharmacokinetic and pharmacodynamic conditions, and it is necessary to select an optimal sample for analysis, which is currently plasma in most cases. The benefit of therapeutic monitoring of a drug must also be proven by a randomised prospective study. Therapeutic monitoring of antitumor drugs appears to be an excellent way of individualising therapy in order to prolong and improve the quality of life of oncology patients, but its introduction into clinical everyday life is faced with numerous obstacles. Many see the future of antineoplastic TDM in the development of simpler monitoring methods, with the relief of the health system by developing sampling techniques that the patient can carry out at home, or through different biosensors that would even enable continuous monitoring of drug concentrations. Finally, the opening of medicine to new technologies and the involvement of clinical pharmacists, as well as public pharmacists in the multidisciplinary oncology team will surely ease the implementation of therapeutic drug monitoring in the clinical practice, thereby improving the therapeutic outcomes and increasing the therapeutic efficacy, while reducing the number of adverse events related to antitumor therapy.