A network node configured for use as an Access and Mobility Management Function, AMF, (300, 104) of a telecommunications network, the AMF for controlling network slice assignment to a user equipment, UE, within the telecommunications network, and comprising: a receiver configured to receive a UE request originating from the UE, the UE request being any one of a registration request and a session request and comprising a UE-provided network slice selection assistance information, NSSAI; a network slice requester configured to control a transmitter to transmit a network slice selection request to a Policy Control Network Slice Selection function, PC_NSS, (400, 108), wherein the receiver is configured to receive, from the PC_NSS, network slice data comprising an authorized NSSAI identifying a network slice assignable for the UE, and a corresponding network slice routing rule identifying the assignable network slice and a set of network functions forming the assignable network slice; and a network slice manager configured to control assignment of a network slice to the UE in dependence on the authorized NSSAI and network slice routing rule received from the PC_NSS as follows: if the authorized NSSAI matches the UE-provided NSSAI, the network slice manager is configured to enforce the network slice routing rule received from the PC_NSS for the UE-provided NSSAI; if the authorized NSSAI does not match the UE-provided NSSAI, the network slice manager is configured to determine whether the authorized NSSAI is supported by the AMF (300, 104); if the authorized NSSAI is not supported by the AMF, the network slice manager is configured to control the transmitter to transmit the UE request to a further AMF for control of assignment of a network slice for the UE; and if the authorized NSSAI is supported by the AMF, the network slice manager is configured to enforce the network slice routing rule received from the PC_NSS for the authorized NSSAI. a receiver configured to receive a UE request originating from the UE, the UE request being any one of a registration request and a session request and comprising a UE-provided network slice selection assistance information, NSSAI; a network slice requester configured to control a transmitter to transmit a network slice selection request to a Policy Control Network Slice Selection function, PC_NSS, (400, 108), wherein the receiver is configured to receive, from the PC_NSS, network slice data comprising an authorized NSSAI identifying a network slice assignable for the UE, and a corresponding network slice routing rule identifying the assignable network slice and a set of network functions forming the assignable network slice; and a network slice manager configured to control assignment of a network slice to the UE in dependence on the authorized NSSAI and network slice routing rule received from the PC_NSS as follows: if the authorized NSSAI matches the UE-provided NSSAI, the network slice manager is configured to enforce the network slice routing rule received from the PC_NSS for the UE-provided NSSAI; if the authorized NSSAI does not match the UE-provided NSSAI, the network slice manager is configured to determine whether the authorized NSSAI is supported by the AMF (300, 104); if the authorized NSSAI is not supported by the AMF, the network slice manager is configured to control the transmitter to transmit the UE request to a further AMF for control of assignment of a network slice for the UE; and if the authorized NSSAI is supported by the AMF, the network slice manager is configured to enforce the network slice routing rule received from the PC_NSS for the authorized NSSAI. if the authorized NSSAI matches the UE-provided NSSAI, the network slice manager is configured to enforce the network slice routing rule received from the PC_NSS for the UE-provided NSSAI; if the authorized NSSAI does not match the UE-provided NSSAI, the network slice manager is configured to determine whether the authorized NSSAI is supported by the AMF (300, 104); if the authorized NSSAI is not supported by the AMF, the network slice manager is configured to control the transmitter to transmit the UE request to a further AMF for control of assignment of a network slice for the UE; and if the authorized NSSAI is supported by the AMF, the network slice manager is configured to enforce the network slice routing rule received from the PC_NSS for the authorized NSSAI.