Shadow IT—unauthorized information systems developed and operated outside formal governance structures—has evolved from episodic workaround to structural feature of banking technology landscapes, yet organizations struggle to rationalize these informal tools despite repeated modernization efforts. This thesis examines why shadow IT persists in legacy-intensive banking environments and how automation-driven rationalization initiatives reshape rather than resolve underlying governance challenges. Through action research conducted during a six-month consulting intervention in a major Italian banking institution, the study investigates shadow IT rationalization across four operational domains governing revenue provisioning, contract management, succession workflows, and regulatory reporting. These processes relied on Excel macros and Access databases containing undocumented business logic accumulated over decades.