The deep underground MACRO detector is currently operating at the Laboratori Nazionali del Gran Sasso in Abruzzo, Italy. MACRO has a geometrical acceptance of nearly 10000 m^2 at an average depth of 3.8 kilometers of water equivalent under the mountainous overburden of the Gran Sasso d'Italia. We use this large area detector to research several topics. The specialty of MACRO is the search for magnetic monopoles: particles with bare north or south magnetic charge. These particles are a natural consequence of Grand Unified Theories, which also predict that the monopole will be very massive: perhaps 10^16 GeV. Such particles can only be produced by the intense energies available during the big bang. MACRO operates like a giant Time-Of-Flight counter to detect the unique signature of a slow moving but penetrating massive particle. MACRO is equipped with tanks of liquid scintillator, planes of streamer tubes and plates of track etch material to hopefully record a convincing signature from a single candidate event. MACRO's high resolution tracking and timing are also used to perform high statistics measurements of cosmic ray muons; in particular the scintillator timing is used to distinguish upward going muons produced by neutrino interactions in the rock. This is an opportunity to investigate the possible flavor oscillation of massive neutrino's as suggested by the atmospheric neutrino puzzle, as well as possible astrophysical point sources of neutrinos. The large area and fine grain of the tracking affords unique measurements of large multiplicity muon showers- from these we hope to infer information about the nature of the primary particles in the high energy range of the cosmic ray spectrum. The large mass of liquid scintillator (~ 600 tons) is also instrumented to identify a burst of low energy pulses as might be caused by the flux of neutrinos from a supernova within our galaxy.