The intention of this course is to give an overview of the main theoretical physics concepts that underlie our understanding of high-energy collisions, specifically proton-proton collisions at the LHC machine. To be more precise, the emphasis is on QCD aspects of collisions, which by necessity enter in all processes at a hadron collider. It is complementrary to other Nordic schools, such as the recent Odense one on BSM physics at the LHC and the upcoming Ph.D. course in Copenhagen on Advanced methods in statistical data analysis. The lectures are interspersed and complemented by exercise sessions, with both analytical and computer-based tasks. These tasks will require the students to run some software on their own computer.