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 of a conversation 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; one or more processors configured to run a client application having a user interface capable of operating in a conversation mode to display in sequence transmitted and received messages of the conversation in the available display area or in an engagement mode to display one or more of the messages, each displayed message in the engagement mode occupying a larger respective proportion of the available display area than in the conversation mode; wherein the client application is configured to detect a condition indicative of a level of engagement of the user between a lower level of engagement and a higher level of engagement by analyzing the user's activity at the user device and responsive to the detected condition to automatically modify the client user interface, when configured to operate in one of said modes, to operate in the other of said modes instead, wherein the detected condition is based on at least one of: a detected frequency at which the messages are transmitted and received between the user device and the communication network; a presence status of the user, the presence status indicating a current state of the user, the current state of the user comprising the user's activity at the device; historic user data; and the user's past manual switching behaviour. 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 of a conversation 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; one or more processors configured to run a client application having a user interface capable of operating in a conversation mode to display in sequence transmitted and received messages of the conversation in the available display area or in an engagement mode to display one or more of the messages, each displayed message in the engagement mode occupying a larger respective proportion of the available display area than in the conversation mode; wherein the client application is configured to detect a condition indicative of a level of engagement of the user between a lower level of engagement and a higher level of engagement by analyzing the user's activity at the user device and responsive to the detected condition to automatically modify the client user interface, when configured to operate in one of said modes, to operate in the other of said modes instead, wherein the detected condition is based on at least one of: a detected frequency at which the messages are transmitted and received between the user device and the communication network; a presence status of the user, the presence status indicating a current state of the user, the current state of the user comprising the user's activity at the device; historic user data; and the user's past manual switching behaviour. a detected frequency at which the messages are transmitted and received between the user device and the communication network; a presence status of the user, the presence status indicating a current state of the user, the current state of the user comprising the user's activity at the device; historic user data; and the user's past manual switching behaviour.