iMessage was announced by Scott Forstall at the WWDC 2011 keynote on June 6, 2011. A version of the iOS Messages application with support for iMessage was included in the iOS 5 update on October 12, 2011.
On February 16, 2012, Apple announced that a new OS X Messages application with support for iMessage, replacing iChat, would be part of OS X Mountain Lion. Mountain Lion, with Messages, was released on July 25, 2012.
On October 23, 2012, Apple CEO, Tim Cook announced that Apple device users have sent 300 billion messages using iMessage and that Apple delivers an average of 28,000 messages per second.
On November 12, 2012, Chetan Sharma, a technology and strategy consulting firm, published the US Mobile Data Market Update Q3 2012, noting the decline of text messaging in the United States, and suggested the decline may be attributed to Americans using alternative free messaging services such as iMessage.
iMessage allows users to send texts, documents, photos, videos, contact information, and group messages over Wi-Fi, 2G, 3G or LTE to other iOS or OS X users, thus providing an alternative to standard SMS/MMS messaging for most users with devices running iOS 5 or later.
iMessage is accessible through the Messages app on an iPhone, iPad or iPod touch running iOS 5 or later or on a Mac running OS X Mountain Lion or later. Owners of these devices can register one or more email addresses with Apple, and, additionally, iPhone owners can register their phone numbers with Apple, provided their carrier is supported. For iPhone users who have an active data connection, Messages will check with Apple if the recipient has iMessage set up. If they do, it will seamlessly transition from SMS to iMessage.
In Messages, a user can see if the other iMessage user is typing a message. A pale gray ellipsis appears in the text bubble of the other user when a reply is started. It is also possible to start a conversation on one iOS device and continue it on another. iMessage-specific functions operate only between machines running iOS 5 or later or running Mountain Lion or later, but, on the iPhone, Messages can use SMS to communicate with non-iOS devices, or with other iPhones when iMessage is unavailable. On iPhones, green buttons and text bubbles indicate SMS-based communication; on all iOS devices, blue buttons and text bubbles indicate iMessage communication.
All iMessages are encrypted and can be tracked using delivery receipts. If the recipient enables Read Receipts, the sender will be able to see that the recipient has read the message.
iMessage also allows users to set up chats with more than two people - a "group chat". However, the group chatting features do not integrate very well with members in the group who have a different type of phone.
If the iPhone is running iOS 5 or greater, the messaging app will send text messages as an iMessage instead of the usual text message. This means that if you are sending text messages with another iOS 5 user, there is no SMS charge associated with the messaging. It is merely treated as an additional data transfer.
iMessage is very similar to MMS: it not only allows the user to send plain text, but also allows the user to send pictures, movies, locations, and contacts.
Just like APNS it sets up a Keep-Alive connection with the Apple servers. Every connection has its own unique code, which acts as an identifier for the route that should be used to send a message to a specific device. The connection is encrypted with TLS using a client side certificate, that is requested by the device on the activation of iMessage.
iMessages are protected by end-to-end encryption 'so no one but the sender and receiver can access them' and Apple assures that '[They] cannot decrypt the data' and '[They do] not log messages'. 
- "iOS 5 - See new features included in iOS 5". Apple. Retrieved October 10, 2011.
- "OS X Mountain Lion - Inspired by iPad. Made for the Mac.". Retrieved February 18, 2012.
- Zach Epstein (October 23, 2012). "Apple Kicks Off iPad Mini Event: 3 Million New iPods Sold, iOS 6 Now On 200 Million Devices". Boy Genius Report. Retrieved February 17, 2013.
- Suzanne Choney. "Text messaging is on decline in US, says report". NBC News. Retrieved February 17, 2013.
- MG Siegler (June 6, 2011). "Apple Has Finally Stuck A Dagger Into SMS. I Love It.". TechCrunch. Retrieved February 17, 2013.
- Apple. "New Version of iOS Includes Notification Center, iMessage, Newsstand, Twitter Integration Among 200 New Features". Retrieved October 1, 2013.
- iPhoneTextClone. "What is an iMessage?". Retrieved May 15, 2013.
- "Inside Apple's move to open up SMS-style messaging to non-mobile clients". Retrieved November 1, 2011.
- "IMessage - IMFreedom Wiki". Retrieved May 2, 2012.