This lecture covers the transition from historical records to modern time-domain surveys, emphasizing the importance of time-domain mindset in astrophysics. Topics include the development of surveys, the significance of variability in astronomy, and the impact of time-domain observations on recent Nobel prizes. The course aims to inspire students with the applications of variability in astrophysics and cosmology, providing insights into time-domain data analysis techniques and the landscape of time-domain surveys. Exercises and presentations are part of the course setup, encouraging engagement with cutting-edge research.