This compilation is a reprint of the Special Issue "Therapeutic drug monitoring and pharmacokinetics-based individualization of drug therapy" of the peer-reviewed journal Pharmaceutics (ISSN: 1999-4923). The primary aim of this Special Issue was to provide a platform for presenting the findings of research performed using novel computer software-based approaches for the pharmacokinetic interpretation of the results of therapeutic drug monitoring. The compilation involves nineteen research articles and five review papers published between 2022 and 2024. The areas covered include pharmacotherapy following organ transplant, the treatment of the severely or even critically ill using antiinfectives, precision oncology, mental illness, cystic fibrosis, anticoagulant therapy, the treatment of diabetes, and ophthalmology.