Theranostics is a field in biomedical research with the goal of developing systems that enable simultaneous detection of a disease, its treatment and monitoring. In the first part of this thesis, we give a thorough review of up-to-date development of theranostics. Then, two theranostic approaches that target altered enzymatic activity in malignant cells are investigated. We introduce RAFT scaffolds for the construction of Cyclopeptidic Chemotherapeutic Prodrugs (cPCPs) in which doxorubicin is bound via a cathepsin B-cleavable peptide linker. Then, we present an extensive in vitro study of new stable phosphatase-sensitive derivatives of 5-aminolevulinic acid (5-ALA) which take advantage of the aberrant heme synthesis found in cancer cells, and may also profit from an upregulated activity of alkaline phosphatases that has been found underlying various cancer types. Finally, a new concept of 5-ALA conjugation to squalene, a natural cholesterol precursor, is explored in another in vitro study on cancer cells.