Cooperative diversity has emerged as a promising technique to combat fading and improve reliability in a wireless environment. In cooperative diversity protocols, neighboring nodes act as virtual multiple-input-multiple-output (VMIMO) systems, where they cooperate with the transmitter-receiver pair to deliver multiple copies of a packet to the receiver via spatially independent fading channels. These multiple copies of the same packet can be combined at the receiver to recover the original packet. Medium Access Control (MAC) protocols play an important part in realizing this concept by effectively coordinating handshake and transmissions between source, partner and destination nodes. In this thesis, we investigate opportunities for improving reliability in Wireless Sensor Networks using cooperative MAC protocols. First, a Medium Access Control protocol, called CPS-MAC, is proposed. Design challenges such as efficiently waking up neighborhood nodes, minimizing energy overhead, and partner selection are also addressed. Then, Reliable Cooperative Transmission-MAC (RCT-MAC) is proposed which extends the functionality of Cooperative Preamble Sampling-MAC (CPS-MAC) by implementing the Cooperation on Demand concept: nodes cooperate only when needed. Furthermore, RCT-MAC is one of the first attempts to compare the performance of a cooperative Wireless Sensor Network (WSN) MAC protocol against conventional protocols for WSNs namely B-MAC, L-MAC, and IEEE 802.15.4. The reliability vs energy efficiency tradeoff is analyzed for both CPS-MAC and RCT-MAC. Lastly, we evaluate a Packet Error Prediction scheme particularly envisioned for preamble sampling cooperative protocols and meant to supplement traditional partner selection schemes. The correlation between the handshake packets and data packets is analyzed using empirical data.