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One of the fundamental goals guiding research in the biological sciences is to understand how cellular systems process complex physical and environmental cues and communicate with each other across multiple length scales. Importantly, aberrant signal processing in these systems can lead to diseases that can have devastating impacts on human lives. Biophysical studies in the past several decades have demonstrated that cells can respond to not only biochemical cues, but also to mechanical and electrical ones. Thus, the development of new materials that can both sense and modulate all of these pathways is necessary. Semiconducting nanowires are an emerging class of discovery platforms and tools that can push the limits of our ability to modulate and sense biological behaviors for both fundamental research and clinical applications. These materials are of particular interest for interfacing with cellular systems due to their matched dimension with subcellular components (e.g., cytoskeletal filaments), and easily tunable properties in the electrical, optical and mechanical regimes. Rational design via traditional or new approaches, e.g., nano-casting and mesoscale chemical lithography, can allow us to control micron- and nano-scale features in nanowires to achieve new biointerfaces. Both processes endogenous to the target cell and properties of the material surface dictate the character of these interfaces. In this thesis, I describe my work on the (1) synthesis and characterization of a silicon nanowire material and (2) use of that material in different configurations for the optical modulation of three cellular systems. In the first section, I discuss the synthesis of coaxial p-type/i-type/n-type silicon nanowires (PIN-SiNWs) with enhanced surface atomic Au, which allows for the efficient production of photoelectrochemical currents upon 532 nm laser illumination. Here, I also describe the adaptation of patch clamp electrophysiology for the measurement of photocurrents from single nanowires in an interconnect-free configuration. In the second section, I first use single free-standing PIN-SiNWs to optically modulate the excitability of single primary dorsal root ganglion neurons. I demonstrate that this neuromodulation is occurring an atomic Au mediated photoelectrochemical process, rather than a photothermal one. Next, I incorporate PIN-SiNWs into a SU-8 polymeric grid structure fabricated via photolithography and use this mesh structure (SU-8-PIN mesh) to optically train neonatal rat ventricular cardiomyocytes as well as adult rat hearts ex vivo to beat at a target frequency. In these experiments, the cardiomyocytes are cultured atop the mesh or the mesh is stuck onto the adult hearts in the absence of a suture or adhesive via capillary forces. A moving laser stimulus is used to train the cardiomyocytes in both cases in order to mimic physiological stimuli that interact with cells not just at a single point but spatially all over the cell. In the last case, I label PIN-SiNWs with fluorescently conjugated streptavidin and treat primary mouse T cells with a biotinylated anti-CD45 antibody that allows for the generation of T cell-PIN-SiNW complexes. I develop a method for the optical stimulation of populations of these T cells while they are being activated through their T cell receptors. I show that depolarizing populations of T cells optically via a PIN-SiNW mediated process during T cell activation dampens TCR signaling, as demonstrated via intracellular phospho-flow cytometry. In this thesis, I demonstrate non-invasive, non-genetic optical modulation of various cellular systems using PIN-SiNWs in a free-standing configuration that can be dispersed in a drug-like fashion, or in a substrate configuration as a high density mesh, or lastly as a free-standing complex with the target cell. This work in neurons and cardiomyocytes has implications for photo-responsive therapeutics in the context of diseases in excitable cell types that are characterized by aberrant electrical activity. The work in T cells, while also having potential for use in autoimmune therapeutics, also helps us to understand a more fundamental question of how membrane voltage affects T cell activation. Moreover, this work is an example of a novel technique that can be used to bridge electrical cellular signaling with other signaling pathways in populations of non-excitable cells.