In the first part of the course we learnt that if we allow market forces to work we reach an efficient outcome: the maximum benefit that can be generated by a market. The second part of the course explores cases where the markets fail to accomplish our goals. This week sets up the benchmark case of the perfectly competitive market: a model we will modify in the next few weeks. We define Perfect Competition, learn to model it graphically and discuss some key results in terms of long run profits and implications for efficiency.