Systems get more and more complex, and depend increasingly on software; so how do we demonstrate their safety? It appears that many of the proven methods may no longer be fit for purpose, because they often depend on the assumption that if all components function as intended, the system will be safe. In complex software-intensive systems, the interaction between components becomes critical, rather than their individual behaviour. This demands a systems approach to safety, with a multi-faceted analysis of its properties. In this book, we explore the challenges of the introduction of novel technology, aiming to find new approaches to demonstrating that these complex software-intensive systems are safe. We use the introduction of electric Christmas trees as an example, because they challenge the existing legislation and mindset, which is often still based on conventional electro-hydraulic technology. What kind of evidence do we need to convince stakeholders that electric Christmas trees are as safe as the old ones? How do we convince ourselves that the software does not exhibit unwanted interactions?