I'm using the validation rule below to check if the billing period field is blank when the opportunity stage is Compliance and it works perfectly, but 
I actually have 3 more pick list values (Conditions Met, Funded and No Sale) I'd to check if the billing period is blank. What is the syntax for this? I've done many searches and can't find an answer. I feel like this is so simple. Thanks for your help.

ISPICKVAL(Stage__c,"Compliance") 
&& ISBLANK(Billing_period__c)

I'm using the validation rule below to check if the billing period field is blank when the opportunity stage is Compliance and it works perfectly, but 
I actually have 3 more pick list values (Conditions Met, Funded and No Sale) I'd to check if the billing period is blank. What is the syntax for this? I've done many searches and can't find an answer. I feel like this is so simple. Thanks for your help.

ISPICKVAL(Stage__c,"Compliance") 
&& ISBLANK(Billing_period__c)