A user device comprising: a display having an available display area; a network interface configured to transmit and receive messages between the user device and a communication network in an instant messaging communication session, the messages constituting a sequence of messages in which a user of the user device is engaged by instigating transmission of new messages in response to receipt of received messages in the communication session; an access component for accessing computer storage configured to store the transmitted and received messages; and one or more processors configured to run a client application having a user interface configured to operate in a conversation mode to display in the available display area at least some of the transmitted and received messages in sequence, characterised by the client application configured responsive to a mode switch instruction to access the computer storage to automatically identify the most recently received message of the stored messages in said communication session and to modify the client user interface to operate in an engagement mode to display at least the identified message; whereby the mode switch instruction is instigated automatically by the client application in response to determining a level of engagement of the user in the conversation based on at least one of: a current frequency at which the messages of the instant messaging communication session are being transmitted and received, a presence indicator of the user, historic user data, or the user's past manual switching behaviour; whereby each displayed message (402, 404) in the engagement mode occupies a larger respective proportion of the available display area than in the conversation mode. a display having an available display area; a network interface configured to transmit and receive messages between the user device and a communication network in an instant messaging communication session, the messages constituting a sequence of messages in which a user of the user device is engaged by instigating transmission of new messages in response to receipt of received messages in the communication session; an access component for accessing computer storage configured to store the transmitted and received messages; and one or more processors configured to run a client application having a user interface configured to operate in a conversation mode to display in the available display area at least some of the transmitted and received messages in sequence, characterised by the client application configured responsive to a mode switch instruction to access the computer storage to automatically identify the most recently received message of the stored messages in said communication session and to modify the client user interface to operate in an engagement mode to display at least the identified message; whereby the mode switch instruction is instigated automatically by the client application in response to determining a level of engagement of the user in the conversation based on at least one of: a current frequency at which the messages of the instant messaging communication session are being transmitted and received, a presence indicator of the user, historic user data, or the user's past manual switching behaviour; whereby each displayed message (402, 404) in the engagement mode occupies a larger respective proportion of the available display area than in the conversation mode. a current frequency at which the messages of the instant messaging communication session are being transmitted and received, a presence indicator of the user, historic user data, or the user's past manual switching behaviour; whereby each displayed message (402, 404) in the engagement mode occupies a larger respective proportion of the available display area than in the conversation mode.