This paper addresses the consensus problem in a multi-agent system with an active leader, where the leader's state variables, such as velocity and acceleration, are not measurable. The authors propose a decentralized control scheme that includes a neighbor-based local controller and a neighbor-based state-estimation rule for each agent. The control law ensures that each agent can follow the leader if the leader's input is known, and the tracking error can be estimated if the input is unknown. The analysis is based on a Lyapunov function, and the results are proven for both connected and non-connected interconnection graphs. The paper also discusses the extension to more complex leader dynamics and future research directions.This paper addresses the consensus problem in a multi-agent system with an active leader, where the leader's state variables, such as velocity and acceleration, are not measurable. The authors propose a decentralized control scheme that includes a neighbor-based local controller and a neighbor-based state-estimation rule for each agent. The control law ensures that each agent can follow the leader if the leader's input is known, and the tracking error can be estimated if the input is unknown. The analysis is based on a Lyapunov function, and the results are proven for both connected and non-connected interconnection graphs. The paper also discusses the extension to more complex leader dynamics and future research directions.