Charter schools have become an important part of the public education landscape in New York City. In the 2017-18 school year, there were 227 charters educating 114,000 students, about 10% of the city's schoolchildren. The strong academic achievement of students in these schools, as well as parental demand, points to the need for more charter schools. One big impediment is lack of space. Former mayor Michael Bloomberg championed charter schools and accelerated their growth via colocation, the granting of free space in traditional public school buildings. However, during his 2013 campaign for mayor, Bill de Blasio pledged to curtail the practice. In response, in April 2014 the New York State legislature began requiring the city to offer rental assistance to new charters that are denied space in public school buildings. This report examines the de Blasio administration's record regarding colocations, the extent to which there is space available for charters in underutilized public school buildings, and what additional steps the city and state might take to find room for charters. Key findings include: (1) Enrollment in New York City charter schools continues to increase. However, the colocation of new or expanded charter schools in public facilities has slowed dramatically: in the last five years of the Bloomberg administration, 150 charter colocations were approved, or 30 per year; in the first five years of the de Blasio administration, 59 colocations were approved, or about 12 per year. (2) More charters could be accommodated in underutilized public school buildings, especially in neighborhoods with many low-performing schools: in 2016-17, 192 buildings across the city had more than 300 empty classroom seats; 72 buildings had more than 500 empty seats. (3) The cost of the lease-assistance program that helps charters to gain access to private space is growing rapidly: $51.9 million in fiscal year 2018 and likely rising to $62 million in fiscal year 2019. Some of New York City's community school districts-- notably, those with struggling schools where many charters wish to open--enjoy substantial amounts of underutilized space. There are at least three reasons that charters should be offered such space. First, they are public schools. Second, they improve life outcomes for tens of thousands of students, many from poor families, who would otherwise be trapped in low-performing traditional public schools. Third, New York City would benefit financially from doing so: it pays nearly $52 million per year for charters to access private space, and that cost could easily double over the next few years if the city continues to reject a high percentage of charter colocation requests.