What are the most important portfolio management theories you must know in order to competently manage an investment portfolio?
In order to keep the topic focused, I would like to narrow down the set of potential theories to theories that are aimed at the act of portfolio management itself. For example, knowing Keynesian economics may be "nice" for investing, but Keynesian economics is a rather broad theory that isn't directly aimed at portfolio management.
On the other hand, Modern Portfolio Theory IS a theory that is directly aimed at investment/portfolio management.