Ensuring food security for a human population growing towards 10 billion under the pressures of demographic, social and climate change is a huge challenge. Advances across all terrestrial and aquatic food sectors are necessary for more efficient and sustainable production. Agriculture produces the majority of human food, but capture fisheries are crucial for the nutrition and livelihoods of many, and aquaculture will necessary if we are to supply increasing global demands for fish. However, research and policy directed toward improving food security are largely siloed within food sectors. Using data synthesis and modelling approaches, this thesis explores the range of links and interactions among terrestrial and aquatic food sectors and their significance for sustainable development. I show how terrestrial and aquatic food systems interact through interdependencies for animal feeds, shared space or the flow of natural subsidies, linked human resource use, and feedbacks with the climate. By switching to novel ingredients such as algae or insects, I demonstrate how fed aquaculture can substantially reduce its demand for small pelagic ‘forage’ fish used as feed, even after accounting for trade-offs for fish growth and human health benefits. I address the risks of single-sector perspectives to food systems research by highlighting how the drivers of sudden production losses (such as extreme weather) can unexpectedly displace human resource use, or create linked challenges for adaptation, across land and sea. Further, I illustrate how changing human consumption can shift food demands across the land-sea divide, illuminating challenges, and opportunities for sustainability. Ultimately, this thesis highlights how single-sector approaches to food system research can create blind-spots in our understanding of sustainability and how threats may be propagated or diffused across land and sea.