With the ever-growing understanding of the complexity of disease progression, be it cancer, microbial or other, comes a rapid need for more targeted treatments plans. Despite these positive advancements however, this is an area that is still in vital need of research. Non-targeted therapies are leading to antimicrobial resistance and chemo resistant cancers which is putting strain on healthcare systems and leading to reduced efficacy of traditional treatments. The development of responsive molecular systems for use in medicinal settings should allow for more patient specific treatments lessened side-effects and early diagnosis/intervention, all of which may lead to improved treatment efficacy. This will be achieved by improving site selective activation of sensors and drugs, which will not only lessen damage to healthy cells, but should also result in increased efficacy meaning lower administered dosages. This thesis will focus on the development of responsive molecular systems for use in diverse areas; from a theranostic agent against glioblastoma multiforme to the design and synthesis of a new strategy towards responsive anion binding based on ring-expansion. This thesis will open with a literature review (Chapter 1) outlining the various strategies employed in the development of responsive systems, with particular emphasis on self-immolative modes of action. The review will also include discussion of the functional groups utilised for the targeting of specific stimuli. The application of these motifs will then be considered within the medicinal context, encompassing both imaging and therapeutic systems. Finally, the review will address the role of molecular responsiveness within supramolecular chemistry, particularly in relation to anion recognition and binding. This section will outline current approaches to the design of responsive binders, which represent distinct, yet conceptually related, strategies to those discussed previously. Chapter 2 details the design, synthesis and characterisation of two ROS-responsive self-immolative dendrimers for use as cold atmospheric plasma activated theranostics. This compound shows an ability to release its cargo upon reaction with peroxide, demonstrated using UV/Vis and fluorescence spectroscopy, as well as LCMS analysis. It also shows the ability to induce a ‘switch-on’ in cytotoxicity upon exposure to CAP, with the parent compound remaining non-toxic at high concentrations. Furthermore, confocal microscopy shows a ratiometric change in fluorescence output in UMG-251 cells after CAP treatment, exemplifying its utility as a selective theranostic in this cell model. Chapter 3 then switches to a responsive supramolecular focus, reporting a new strategy for developing responsive anion receptors was established through ring expansion of cyclic squaramides. The optimal scaffold was identified as a medium-sized cyclic squaramide, with side-chain length playing a critical role in enabling controlled intramolecular rearrangements. Using this approach, a series of squaramide–amino acid conjugates were synthesised, and selective ring expansions were achieved under mild conditions. Expanded macrocycles exhibited selective binding towards carboxylate anions, while the unexpanded precursors showed minimal interaction. The binding behaviour varied with macrocycle size, influencing both affinity and stoichiometry. These findings demonstrate that cyclic squaramides provide a versatile and tuneable platform for responsive anion recognition, highlighting the potential of ring expansion strategies in the design of functional supramolecular receptors. Finally, Chapter 4 builds upon the findings of Chapter 3 to enhance the use case of the novel responsive anion binding scaffold and make them more applicable to a biological setting. The aim of this chapter was to modulate the N-terminus protecting groups previously used and employ linker chemistry to conjugate NTR and H2O2 targeting triggers. This chapter outlines the design and synthesis of this second-generation of responsive anion binders and investigates their ability to undergo ring expansion to allow for a ‘switch-on’ in anion binding. The design of a potential sensing approach is also discussed.