Composites are of two essentially different types: (a) those made to achieve a unique combination of properties, usually mechanical properties; (b) composites formed for ease of processing. The archetype of (a) is the fibre or laminated composite. The attainable properties will be reviewed and interesting effects arising from the scale of size of the components discussed, notably crack arrest and thin-film effects. Examples of (b) are polymer-polymer composites and some of the processes for forming ceramics and strong metals. A unified example of (a) and (b) are composites of controlled thermal conductivity.