The 60 GHz frequency band, with its 7 GHz wide unlicensed spectrum, opens up avenues to multi-Gigabit communication. However, the extremely short wavelength (on the order of a few millimeters) results in very high path loss and little diffraction or scattering. To extend range, directional antennas or electronically steerable beam-arrays are used to get directivity gain. This makes links in 60 GHz networks inherently directional and this directivity introduces new challenges in terms of communication link establishment and maintenance. First, the beamwidth of the directional antennas plays an important role in determining the maximum rate, unlike in omni-directional networks. Moreover, 60 GHz links are susceptible to breakage due to misalignment and blockage due to their highly directional nature. In this thesis, I have designed, implemented and evaluated a novel cross-layer protocol, BeamRAP, for adapting the beamwidth of directional antennas and the data rate at the physical layer jointly in 60 GHz directional networks. This joint adaptation is necessary since beamwidth and alignment of directional antennas, are the key determinants of link strength and thereby the data rates. Moreover, misalignment of directional antennas due to nodal mobility or link blockage due to environmental mobility cannot be addressed by existing rate adaptation protocols. Therefore, in BeamRAP, I have implemented a new algorithm for beamwidth adaptation in response to the frequency of blockage and misalignment events, to maximize link throughput. I have also introduced new mechanisms for link breakage detection, and fast recovery to restore links without exhaustive search over all antenna sectors. I have also developed a 60 GHz programmable node and testbed using VubIQ 60 GHz transceivers with WARP baseband, and have conducted an extensive measurement study to collect signal strength traces over-the-air for various LOS, reflection and antenna-misalignment scenario. My experiments under multiple environmental and nodal mobility scenario show that BeamRAP achieves up to 2x gains in throughput as compared to a baseline 802.11ad scheme, which does not implement beamwidth adaptation.