This document is a methods manual for measuring microplastic in the environment. This manual outlines step-by-step instructions for quantifying microplastic in marine environmental samples, including processes to stream line terminology and approaches. The methods described here were determined after careful study and laboratory work conducted through a grant from the NOAA Marine Debris Program to the University of Washington, Tacoma. Project goals sought to streamline the terminology and techniques used to assess microplastic concentrations in marine environmental samples, and to develop laboratory procedures to quantify microplastic particles in marine surface waters and bed sediments, as well as personal care products. An overarching aim was to provide scientists and educators with simple techniques that are reproducible and robust without requiring extensive equipment, and to describe a method that could be easily adopted by groups around the world. Depending on the study aims and environmental collection techniques, these techniques can be used to calculate concentrations of microplastics using a variety of metrics, including per piece, per mass, or per volume. Considering metrics is important for comparing results with other researchers. For guidelines and considerations when conducting microplastic analyses, as well as calculations, see the appendices.