Shift left testing is an agile software development practice that involves integrating testing activities earlier in the development process, as opposed to traditional models like the Waterfall Model where testing is conducted at the end. By adopting this approach, developers and testers can identify and resolve defects sooner, leading to reduced costs and improved product quality. This method emphasizes automated testing to cover a large percentage of functionality and aims to minimize the number of defects that reach production. Shift left testing is contrasted with shift right testing, which focuses on testing post-deployment to ensure performance and usability in the production environment. The article discusses different types and implementations of shift left testing, highlighting its advantages such as faster time to market and enhanced customer satisfaction. Although shifting left requires significant changes in methodology and team collaboration, combining it with shift right testing can optimize the entire software development lifecycle through continuous testing.