Structural, thermal, and optical performance (STOP) analysis is gradually becoming a crucial tool in the development of optical systems in general and astronomical telescopes and instruments in particular. We describe the approach of the European Southern Observatory to STOP analysis at the examples of the phasing and diagnostic station and of the pre-focal station of the European Extremely Large Telescope under construction. Further, we discuss the fundamentals of the estimation of thermal effects on optical performance and how to carry it out with the in-house developed software tool Sensitizer.