The capability to manage complexity is one of the key competencies of system engineers for large IT-solutions. We call a technical system "complex" (in contrast to "complicated") if it is impossible (due to the networked interaction of its components) to predict the behavior of the whole system, even if you know exactly how each of the system components behave. On the other hand, customers expect increasingly high reliability of IT systems as their business is more and more dependent on the proper operation and interoperation of the IT systems. First we show how a network of interactions increases the complexity of the overall-system. Then we analyze the complexity management strategies of our system engineers and present generalized strategies based on examples of large customer projects. The examples demonstrate that a high maturity in managing complexity enables to provide IT solutions of ultra-high reliability even if they are complex solutions in the above defined sense.