This talk will be a reprise of a talk given last spring in the Osher lecture series.

Many of the systems that are important to the functioning of society, behave in unexpected ways because they are complex systems. This makes them more then the sum of their pieces. First we will discuss some of the characteristics of these systems, then we will talk about some of the implications for systems ranging from engineered infrastructures like the communications grid, pipeline networks and the power transmission grid to human systems like the financial system, decision making, conflict and even organizations like universities.