Today, NASA held a press conference on the status of the James Webb Space Telescope, the organization's successor to Hubble, and the news was grim. The observatory was supposed to launch between March and June of 2019. JWST will miss that window; while a specific launch time frame hasn't been established, NASA is currently targeting May 2020.
While the telescope's individual components meet their requirements, contractor Northrop Grumman needs more time to test them, integrate them together and do environmental testing. In order to monitor the telescope's schedule, NASA is creating a Independent Review Board (IRB) to monitor this testing and NASA will take its recommendations into account when determining a specific launch window. That will occur sometime this summer.
Many suspected this announcement was coming after a report from the US Government Accountability Office earlier this month. The GAO found that ongoing technical issues with the telescope meant that launch delays were likely, and that the project was at risk of breaching the $8 billion cap set by Congress, which would mean it would need to be reauthorized. The telescope has already encountered delays, and it's safe to say that more will follow. It's an incredibly complex, detailed and delicate device, after all.