In this special issue of Science, "Composite Materials," three Review articles overview the current applications, constraints and future opportunities for composite materials, those made from two or more materials with significantly different physical or chemical properties. The key environmental concern of composite materials - which are widely used in industry settings from auto manufacturing to electronics - is the difficulty of removing individual components from their structures to enable them to be recycled after their end of service life. In the first Review, Amar Mohanty and colleagues address this challenge, discussing the growing interest in composites made from bio-sourced, recycled, or waste materials. Already, biodegradable composites have shown potential for major uses in sustainable packaging. In the future, redirecting recycled plastic materials destined to go to land-filling for blending in composite applications could reduce dependence on less plentiful materials, the authors say, helping in global sustainability efforts. Material scientists will play a key role in advancing sustainable composite alternatives. In a second Review, Michaela Eder and colleagues highlight the potential to use bio-inspired engineering approaches - which rely on just a few base substances that can assemble into a large variety of structures - to make materials that do not feature unnecessary chemical diversity, thus enabling easier recycling. In the final Review, Ian Kinloch and colleagues focus on composite materials with carbon nanotube and graphene additives, which have long been considered exciting prospects among nanotechnology applications, but where challenges remain. They highlight different approaches that could lead to practically useful nanotube or graphene composites.