The Lawful Interception (LI) network function (NF) provides network operators with the ability to deliver information to legal authorities on specific users in the 5G network that have a warrant assigned to them. The thesis describes the conceptual LI high level architecture, the corresponding network functions within, as well as their roles in the system. The two main LI interfaces used in the implementation, LI X1 and LI X2, are explained in detail. Afterwards, the specific arhitecture of the implemented LI system is analysed, containing an IRI-POI (Intercept Related Information - Point of Interception) embedded into the 5G UDM NF. The IRI-POI receives notification events on AMF registrations in the 5G network. The administrative function (ADMF) provisions POI with details on targets that need to be intercepted and destinations to which intercept related information shall be delivered to (MDF2). Lawful Interception has been developed as a new feature for Enea Software's Subscription Manager product. The feature consists of three applications (POI_X1, POI_EVENT, POI_X2) that make up the POI function. To illustrate the implementation, flow charts and sequential diagrams are used throughout to describe the application flows and showcase message exchanges between network elements in the system, along with Java code snippets that highlight how particular functions have been implemented. The application and database configurations have been specified, together with further displays of interactions between the databases and applications. Lastly, tests that have been created using the Robot framework and used to test the developed LI product, are described in detail. Two simulators for the ADMF and MDF2 network functions have been developed, to simulate external network entities during testing.