This chapter describes several assays of the polymerization of actin. The assays vary widely in terms of their cost, ease of performance, time, sensitivity, and information provided. In some assays the actin solution is undisturbed, and in others the sample is sheared. Some assays use native actin, and others employ actin that has been covalently modified with a label. In capillary viscometry assay, viscosity is measured by the time required for a solution to pass through a glass capillary tube. Other methods to measure viscosity include rotational devices. It is reported that the ultraviolet absorption spectrum of actin filaments is different from that of actin monomers. The difference spectrum was positive from 240 nm to 200 nm, with a relative maximum at about 230 nm. This principle is used for the for analysis of actin. In addition, polymerization of actin can be assayed using flow birefringence which is based on the orientation of actin filaments in fluid flow. The sample is placed between two concentric cylinders, one of which rotates. Laminar flow in the sample tends to align the filaments in the direction of flow. This alignment is detected by refraction of polarized light. Finally, the chapter discusses how to measure various parameters associated with actin polymerization. We have separated into an accompanying chapter assays of the interactions among actin filaments.