A method implemented e.g. in a Lightweight Machine-to-Machine (LwM2M) server, comprises establishing a secure communication session with a client and transmitting an observation request to the client. The observation request identifies a first resource at the client, one or more criteria relating to the first resource to trigger a notification to be transmitted from the client to the server, and one or more resources at the client comprising at least one resource other than the first resource, the value of which is to be included in the notification.