This chapter presents the data on the sequence and structure of viroids and of approaches and results in the analysis of structure in relation to function. A historical perspective is given in the book by T. O. Diener while several reviews provide a detailed coverage of the determination of the sequence, secondary structure, and properties of viroids. These are of importance in defining the conserved and variable features of the viroid molecule, and they may indicate regions that have a role in viroid replication and symptom expression. Viroids are the smallest known pathogens in nature, and to date have been found only in flowering plants. The chapter discusses some approaches to investigation of relation of structure to function: sequence comparisons of naturally occurring sequence variants of the same viroid and site-directed mutagenesis. The overall sequence homology between pairs of viroids as calculated using the method of W. J. Wilbur and D. J. Lipman is provided.