New York City has an estimated resident population of more than 8.5 million (Switzerland 8.4 million) living in a land area of 790 square kilometers (Switzerland 41'285 square kilometers), which makes it the most densely populated city in the United States. More than 600'000 commuters get in and out of the City each day, and more than 58 million tourists visit the Big Apple each year. With over one million buildings, which account for around three quarters of the city’s total energy use and greenhouse gas emissions, New Yorkers spend $15 billion on energy each year. New York meets the energy challenge with forward-thinking policies and initiatives, combining energy efficiency, distributed generation and renewable energy, in order to transform its urban systems and infrastructures. Just as Switzerland, New York is facing a historic and fundamental transformation involving a variety of risks: investment risks, climate change related risks, risks related to new business models based on digitalization, or risks associated with a very rapid technological progress. The fourth edition of the Swiss-US Energy Innovation Days took place in this Mega-City from August 20 to August 23, 2017. We had an exciting 3-day learning journey on how New York and Switzerland are dealing with the challenges, risks and chances of the changing energy world.