So I'm curious which course(s) PF found the most difficult during undergrad, whether it's a math course, physics, engineering or art history! Difficulty is subjective as well as it's definition. It doesn't matter if it was your lowest grade or what you spent the most hours studying for, or some other ranking system entirely. Make sure you explain! Why was it the hardest course for you? What advice do you give to similar students taking the course? Feel free to answer if you haven't graduated yet ~ Personally, it's 3rd-year Quantum Mechanics (still non-relativistic, dirac notation, 3D schrodinger, hydrogen atom). It was more mathematically intense than previous physics courses and Dirac notation confused the heck out of me. I lost sight of it as a physics course and forgot all those kets and operators were physical things. I lost motivation because of this and learning the subject became dull. I took some great advice and started reading different textbooks on the subject (Feynman's Lectures III is my favourite) and after reading like 5 different sources, I finally started to understand why things were being done the way they were. It seemed to me that each author was leaving something out that didn't complete the picture for me, and it required all of them together to do it. I came out loving the subject with more motivation than ever.