MAC and routing protocols are often designed for single-input-single-output (SISO) wireless systems, where each node is equipped with a single antenna for transmission and reception (typically, operated in a half-duplex manner). Significant improvement in performance can be achieved by employing multi-input multi-output (MIMO) techniques, whereby multiple transmit and/or receive antennas are used to provide spatial diversity. MIMO offers three types of gains: array, diversity, and multiplexing. The array gain is achieved either at the transmitter through directional alignment of the transmitted signal or at the receiver by coherently combining independently received copies of the signal. Diversity gain is interpreted as the slope of the average BER curve versus SNR, which is proportional to the number of independent paths. Multiplexing gain is obtained when different signals are transmitted over various transmit antennas for the purpose of increasing the total transmission capacity of the link. In this project, we are investigating the feasibility of adapting the number of transmit/receive antennas (on a per-packet basis) in multi-antenna wireless devices for the purpose of minimizing energy consumption and/or maximizing network capacity. Our initial setup considers networks with 2-antenna nodes, allowing for four possible transmit/receive antenna configurations (also called "antenna modes") per active link: 1-by-1 (SISO), 2- by-1 (MISO), 1-by-2 (SIMO), and 2-by-2 (MIMO). Depending on the type of MIMO gain (diversity vs. multiplexing) and the primary performance objective (energy reduction vs. throughput), we have developed several MIMO-adaptive MACs for MANETs and wireless LANs. These include E-BASIC, MIMO-POWMAC, and CMAC. E-BASIC targets MIMO systems that are capable of providing diversity gain. It allows nodes to distributively adapt their antenna modes, transmission powers, and modulation orders (for multi-rate systems) on a per-packet basis such that the total per-bit energy requirement (transmission plus circuit) is minimized. The protocol was integrated into the design of a power-aware routing (PAR) scheme for MIMO-capable ad hoc networks. In contrast of E-BASIC, MIMO-POWMAC is aimed at multi-antenna systems that offer multiplexing gain.