Nanoparticles have generated much excitement as a result of their often unique properties, inherently dependent on nanoparticle material, shape and size. Virtually all conceivable nanoparticle applications will require excellent control over how nanoparticles are assembled and linked to other components. When several nanoparticles are brought together, the assembly structure is crucial in determining their newly emergent properties. However, the synthetic chemistry techniques required to control nanoparticle functionalisation and assembly are still under-developed, with complex biological or supramolecular systems being the current best approaches. There remains a need for simple, generalisable strategies for molecular-level control over nanoparticle functionalisation and assembly. This thesis presents the development of a toolkit of nanoparticle building blocks, which may be assembled in a predictable and controlled way, governed by simple and easily optimised abiotic molecular systems. Efficient, size-controlled, direct synthesis of functionalised gold nanoparticle building blocks with control over size and dispersity is developed. ¹⁹F NMR spectroscopy studies provide a fundamental understanding of the implications of confinement at the nanoparticle surface for molecular reactivity. Two self-assembly strategies, each resulting in structures of high order and predictability, are presented. First, the reversible nature of dynamic covalent boronic ester formation is exploited to induce reversible nanoparticle self-assembly. Links between molecular details and resulting morphology are demonstrated and rationalised. A second strategy exploits multivalent non-covalent interactions, resulting in ‘planet–satellite' structures displaying high order, stability and predictability. This thesis demonstrates that relatively simple molecular systems present a viable, and ultimately more flexible, alternative to existing methods of directing precise, predictable control of nanoparticle functionalisation and assembly. Advancing a molecular-level understanding of the underlying processes enables a high level of control. Future application of this molecular approach to dynamic nanomaterial control will lead to more complex and sophisticated nanostructures, helping nanotechnology progress towards its undoubtedly revolutionary full potential.