What is interoperability?  

According to ISO/IEC 2382-01, Information Technology Vocabulary, Fundamental Terms, interoperability is defined as follows: "The capability to communicate, execute programs, or transfer data among various functional units in a manner that requires the user to have little or no knowledge of the unique characteristics of those units”, according to ISO/IEC 2382-01, Information Technology Vocabulary, Fundamental Terms.

For IHE, IT systems are interoperable, if they can properly exchange clearly defined sets of relevant information in the context of a specific clinical situation and perform appropriate actions as described by the IHE specifications.

So why is IHE needed?
There are hundreds of standards for health information exchanges, written to support a wide range of clinical processes and include many optional features. To obtain interoperability with regard to a specific clinical task IHE  creates profiles of relevant standards that make essential features for supporting the clinical task mandatory for those products for which conformance with a profile is claimed. IHE profiles specify the information that must be exchanged between systems and the actions that recipient systems must take on receipt of the information. These IHE Integration Profiles do not constrain the way in which systems are designed and help to enhance to ease of use.