Age-of-information (AoI) is used to characterize the freshness of information, and is critical for information monitoring, tracking, and control, which is typically required in many network applications, such as autonomous vehicles, virtual/augmented reality, and Internet-of-Things (IoT). Both inter-arrival times and delays of packets affect AoI performance, and thus traditional delay-efficient algorithms do not necessarily exhibit low AoI performance. This calls for “age-efficient” algorithm design in communication networks, which forms the focus of this chapter. In particular, we first discuss the recent advances in the age-efficient algorithm design for three different types of common network traffic: (i) elastic traffic (cf. Section 1.1): packets are allowed to be delivered without any specific deadline constraints; (ii) inelastic traffic (cf. Section 1.2): packets will be dropped if they are not delivered within a specific deadline; (iii) heterogeneous traffic (cf. Section 1.3): different packets may have different size. To facilitate our discussions, we explicitly consider the discrete-time model and emphasize the difference between age-efficient and delay-efficient algorithm design paradigms. Then, we examine “fresh” scheduling design for remote estimation with the goal of optimally balancing the trade-of between the estimation accuracy and the communication cost (cf. Section 1.4).