Small particles are ubiquitous in the natural and built worlds and have tremendous impact throughout. However, a lack of understanding about the properties and chemical composition of small particles limits our ability to predict, and control their applications and impacts. Challenges in Characterizing Small Particles: Exploring Particles from the Nano- to Microscales summarizes presentations and discussions at a 2010 National Academies roundtable. Speakers at this roundtable discussed the crucial types of information that need to be determined about small particles in different media. They also explored the critical importance of small particles in environmental science, materials and chemical sciences, biological science, and engineering, and the many challenges involved in characterizing materials at the nano- and microscales. The discussions on characterization included static, dynamic, experimental, computational, and theoretical characterization. The workshop also included several research tool presentations that highlighted new advances in characterizing small particles.