PRIMER Transcription produces an RNA transcript, which contains the entire nucleotide sequence as it is imprinted in the DNA. In other words, the primary RNA transcript also contains the intronic sequences. These intronic sequences must be removed to create a continuous coding sequence that will be able to be translated to a protein. The process of intron removal is called splicing. However, not all introns are spliced-out the same way. Depending on the mechanism involved in their splicing, the introns are divided into three groups. In this chapter, I will present the steps of intron removal, emphasizing the 3-D structure of RNAs and proteins involved. Some introns are removed with the help of proteins, but others self-catalyze their cleavage. Therefore, the 3-D structure of the introns must be important in splicing and will be presented in order to visualize the mechanism. The determination of the 3-D structure of self-catalyzed introns is another celebrated example of how the structure reveals the function.