All of the members of the microsporidia possess a unique, highly specialized invasion mechanism that involves the polar tube and spore wall. This chapter reviews the data on the organization, structure, and function of this invasion organelle. The application of immunological and molecular techniques and recent genome sequencing data has resulted in the identification of multiple polar tube and spore wall proteins (SWPs). The interactions of these identified proteins in the formation and function of the polar tube and spore wall remain to be determined. Inside the spore, the polar tube is filled with material and is often termed the polar filament; however, this chapter uses the term polar tube to refer to this structure when it is within the spore as well as when it forms a hollow tube after germination and is found outside the spore. The chapter presents details on the spore activation and discharge.