More than 300 million barrels of saltwater is produced everyday from oil and gas production wells. Most of this volume is injected through either saltwater disposal wells or used for water flooding and enhanced recovery purposes. Usually, the regulations require the injection to be conducted through the injector well tubing that is isolated from the well annulus to protect the underground source of drinking water (USDW) by preventing any possible leak through the well casing. Monitoring of the annulus pressure during injection ensures the well integrity. The annulus pressure changes can occur by one of the following mechanisms: thermal expansion of the annulus fluid; ballooning of the injection tubing; communication between the tubing and the annulus; or fluid migration behind the casing. Determining the communication mechanism can be a complex process and a need may arise to run several testing procedures and inspect all the wellbore components. Successful evaluation of the annulus pressure values and trends can directly identify the root cause of the annulus pressure buildup and simultaneously save time and reduce the cost associated with the workover operations. The seven case studies presented in this paper focus on the details pertaining to the annulus pressure buildup under different well conditions and purposes the interpretation technique for each case.