Characterization of composite microstructure is a crucial area since such features play a major role in processability, performance, damage tolerance, and service life. Moreover, since the reinforcement geometries are becoming more complex, the need to know and understand microstructure has increased significantly. One example is hybrid composites where the use of two or more fiber types add new complexity to the microstructure. In response to this need, NIST is developing or modifying test methods for characterizing local structure at a variety of size scales. Four examples will be briefly discussed. The first is the single fiber fragmentation test which examines the micromechanics of the fiber-matrix interface region. Continued improvement of this test in recent years has made it more reliable and versatile. The second example is a fiber optics method which can study the cure behavior of resins in the fiber-matrix interfacial region. The third example applies microscopy methods to characterize the microstructural features in hybrid composites. Finally, a new technique called optical coherence tomography is applied to composites and permits nondestructive generation of three dimensional images with internal structure and damage. The images can be obtained rapidly and cost effectively with resolution down to 10 μm.