Signal transductions play pivotal roles in cellular functional outputs. Previous studies employed systems such as pharmacological inhibition, RNA interference and overexpression of proteins of constitutive active/dominant negative forms. However, these methods fail to manipulate the signal transductions with high spatial and temporal resolution. Recently, the new technology optogenetics emerged, which overcomes the issues that previous methods have. The authors have developed the Magnet system, which allows us to control cell membrane dynamics spatiotemporally with high spatial and temporal resolution. This chapter introduces the authors' work on the Magnet system from development to applications.