Historically, ad groups in 
Display campaigns have not served until you added at least one positive ad group targeting criterion, such as 
Placement or 
CriterionUserList. This behavior worked well if your goal was to target specific criteria on 
Display, but not if your goal was to target as broad an audience as possible.
What's changing
Starting 
January 11, 2017, a 
Display ad group will be eligible to serve ads as soon as its 
status is set to 
ENABLED and it has a bid, a budget, and an approved ad. If the ad group does not have any ad group level positive targeting criteria, then the ad group will target the 
entire Display network, subject to exclusions and campaign-level targeting settings (such as geo, language, etc.), with the potential to quickly exhaust your campaign budget.
This change 
only impacts campaigns with 
AdvertisingChannelType of 
DISPLAY or 
VIDEO.
Before this feature is enabled on your account, any 
existing Display ad groups without ad group level positive targeting criteria will be set to 
PAUSED.
What you should do
For 
existing Display campaigns and ad groups:
  - If you want to target the entire Display network (subject to exclusions and campaign-level targeting settings), then after the feature is enabled, make sure you set those ad groups to ENABLED, since they will be in thePAUSEDstate.
- Otherwise, add the desired positive targeting criteria to each ad group, and then set it to ENABLED.
For 
new Display campaigns and ad groups, review your application's ad group creation workflow to ensure that it does the following:
  - Sets the AdGroupstatus field toPAUSEDwhen creating a new ad group via AdGroupService.mutate and anADDoperation.
- Only sets the AdGroupstatus toENABLEDafter adding targeting criteria (if desired). You can update the status of yourAdGroupvia AdGroupService.mutate and aSEToperation.
In addition, if your application adds and removes targeting criteria from existing 
Display ad groups, make sure that it will also properly update the the ad group's 
status to 
PAUSED or 
ENABLED if the intended side-effect of criterion addition or removal is to pause or start serving. This is particularly important when removing the only positive criterion for an ad group, since this will change its reach from very narrow to very broad.
If you have any questions, please post on the 
forum or the 
Ads Developers Plus Page.
 - Josh Radcliff, AdWords API Team
 - Josh Radcliff, AdWords API Team