This book reports on a study on physics problem solving in real classrooms situations. Problem solving plays a pivotal role in the physics curriculum at all levels. However, physics students' performance in problem solving all too often remains limited to basic routine problems, with evidence of poor performance in solving problems that go beyond equation retrieval and substitution. Adopting an action research methodology, the study bridges the `research-practical divide by explicitly teaching physics problem-solving strategies through collaborative group problem-solving sessions embedded within the curriculum. Data were collected using external assessments and video recordings of individual and collaborative group problem-solving sessions by 16-18 year-olds. The analysis revealed a positive shift in the students' problem-solving patterns, both at group and individual level. Students demonstrated a deliberate, well-planned deployment of the taught strategies. The marked positive shifts in collaborative competences, cognitive competences, metacognitive processing and increased self-efficacy are positively correlated with attainment in problem solving in physics. However, this shift proved to be due to different mechanisms triggered in the different students.