A giant and rapidly growing slice of the New York City budget does not pay for current services. Instead, it pays for costs that are the legacy of commitments made in the past: debt service, pension contributions, and retiree health insurance. These legacy costs already exceed 20 percent of the budget and will expand by 20 percent to more than $20 billion in annual spending by fiscal year 2020. Fortunately, there are some steps the City can take to deal with this “20-20-20-20” dilemma.