  The study of galaxy protoclusters is beginning to fill in unknown details of
the important phase of the assembly of clusters and cluster galaxies. This
review describes the current status of this field and highlights promising
recent findings related to galaxy formation in the densest regions of the early
universe. We discuss the main search techniques and the characteristic
properties of protoclusters in observations and simulations, and show that
protoclusters will have present-day masses similar to galaxy clusters when
fully collapsed. We discuss the physical properties of galaxies in
protoclusters, including (proto-)brightest cluster galaxies, and the forming
red sequence. We highlight the fact that the most massive halos at high
redshift are found in protoclusters, making these objects uniquely suited for
testing important recent models of galaxy formation. We show that galaxies in
protoclusters should be among the first galaxies at high redshift making the
transition from a gas cooling regime dominated by cold streams to a regime
dominated by hot intracluster gas, which could be tested observationally. We
also discuss the possible connections between protoclusters and radio galaxies,
quasars, and Ly-alpha blobs. Because of their early formation, large spatial
sizes and high total star formation rates, protoclusters have also likely
played a crucial role during the epoch of reionization, which can be tested
with future experiments that will map the neutral and ionized cosmic web. Last,
we review a number of promising observational projects that are expected to
make significant impact in this growing, exciting field.
