Development of tools for targeted genome editing and regulation of gene expression has significantly expanded our ability to elucidate the mechanisms of interesting biological phenomena and to engineer desirable biological systems. Recent rapid progress in the study of a clustered, regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) protein system in bacteria has facilitated the development of newly facile and programmable platforms for genome editing and transcriptional control in a sequence-specific manner. The core RNA-guided Cas9 endonuclease in the type II CRISPR system has been harnessed to realize gene mutation and DNA deletion and insertion, as well as transcriptional activation and repression, with multiplex targeting ability, just by customizing 20-nucleotide RNA components. Here we describe the molecular basis of the type II CRISPR/Cas system and summarize applications and factors affecting its utilization in model organisms. We also discuss the advantages and disadvantages of Cas9-based tools in comparison with widely used customizable tools, such as Zinc finger nucleases and transcription activator-like effector nucleases.