The GESAMP assessment focuses on a category of plastic debris termed ‘microplastics’. These small pieces of plastic may enter the ocean as such, or may result from the fragmentation of larger items through the influence of UV radiation. Section 1 provides an introduction to the problem of microplastics in the marine environment, and the rationale for the assessment. The principal purpose of the assessment is to provide an improved evidence base, to support policy and management decisions on measures that might be adopted to reduce the input of microplastics to the oceans. The GESAMP assessment can be considered as contributing to a more formal Assessment Framework, such as the Driver-Pressure-State-Impact- Response (DPSIR) Assessment Framework, which is introduced in Section 2.