The sea-surface microlayer has often been operationally defined as roughly the top 1 to 1000 micrometres of the ocean surface. There has been considerable new research in this area over the past 5–10 years. The microlayer is known to concentrate, to varying degrees, many chemical substances, particularly those that are surface active, and many organisms live and/or find food there. It is clearly the interface through which all gaseous, liquid and particulate material must pass when exchanging between the ocean and the atmosphere. It also plays a vital role in the transfer of various forms of energy (momentum, heat) between the two media.

It is now recognized that important physical, chemical, and biological processes near the air–sea interface are not restricted to what has been traditionally referred to as the ‘microlayer’, but rather occur over gradients of varying thickness. Above the interface is an atmospheric boundary layer of 50–500 μm, where atmospheric turbulence is much reduced. Below the air-water interface the aquatic surface layer contains a series of sublayers (as described by Jack Hardy in Chapter 11). In this book, we use the term ‘microlayer’ in its operational meaning to refer to roughly the uppermost millimetre of the oceans, where properties are most altered relative to deeper waters. We also utilize the following terminology: a ‘film’ refers to a surfactant-influenced surface and a ‘slick’ refers to a visibly surfactant-influenced surface.