Within the coatings industry, particle sizing is widespread, but the underlying impact of particle size on product properties can often be overlooked. Particle size analysis is carried out routinely for quality control purposes and it also plays an important role in the development of new, competitive product specifications. In this article, the importance of particle size and its impact on various aspects of product quality are examined. Its influence on both the optical and bulk properties of pigments is discussed in order to provide a full understanding of the reasons why particle size analysis has become such a key activity. This is the first in a series of articles that will explore particle characterization as applied within the coatings industry, together with the analytical tools that are available to manufacturers and the benefits that can flow from appropriate, accurate and timely analysis.