This paper presents opportunistic relaying protocols for cooperative communications, focusing on decode-and-forward (DaF) and amplify-and-forward (AaF) strategies under an aggregate power constraint. The authors propose simple, distributed relay selection algorithms that do not require global channel state information (CSI) at each relay or central controller, reducing cooperation overhead. They show that both reactive and proactive opportunistic DaF relaying are outage-optimal, meaning they perform as well as the optimal strategy that uses all potential relays. Additionally, they demonstrate that opportunistic AaF relaying is outage-optimal among single-relay selection methods and significantly outperforms AaF strategies based on equal-power multiple-relay transmissions with local CSI. The findings highlight that cooperation benefits can be achieved even when relays do not actively transmit, as they can act as passive listeners, prioritizing the transmission of a single opportunistic relay. Numerical and simulation results support these theoretical findings, showing significant gains in outage probability and spectral efficiency. Proactive opportunistic relaying is also shown to be energy-efficient, as it allows most relays to enter an idle mode during the source transmission, reducing reception energy costs.This paper presents opportunistic relaying protocols for cooperative communications, focusing on decode-and-forward (DaF) and amplify-and-forward (AaF) strategies under an aggregate power constraint. The authors propose simple, distributed relay selection algorithms that do not require global channel state information (CSI) at each relay or central controller, reducing cooperation overhead. They show that both reactive and proactive opportunistic DaF relaying are outage-optimal, meaning they perform as well as the optimal strategy that uses all potential relays. Additionally, they demonstrate that opportunistic AaF relaying is outage-optimal among single-relay selection methods and significantly outperforms AaF strategies based on equal-power multiple-relay transmissions with local CSI. The findings highlight that cooperation benefits can be achieved even when relays do not actively transmit, as they can act as passive listeners, prioritizing the transmission of a single opportunistic relay. Numerical and simulation results support these theoretical findings, showing significant gains in outage probability and spectral efficiency. Proactive opportunistic relaying is also shown to be energy-efficient, as it allows most relays to enter an idle mode during the source transmission, reducing reception energy costs.