The investigation of hot and compressed nuclear matter by means of heavy-ion collisions requires the development of probes that are sensitive to the time evolution of the nuclear phase-space density. Hard particles, such as energetic photons or mesons, which are produced during the reaction, may provide such probes. In the first part of this article we describe the time evolution of the nuclear many-body system within a nonperturbative transport approach as well as the methods employed for the evaluation of the elementary nucleon-nucleon-particle cross sections. The second part contains a description of the experimental set-ups used for the detection of hard photons in heavy-ion experiments and a discussion of the empirical systematics of data gained so far. In the third part of this article we compare the theoretical predictions for differential particle yields with the available data. Detailed comparisons are presented for hard photons at bombarding energies from 15 MeV/u to 125 MeV/u, for dileptons at 1 GeV/u and for mesons (π's, η's, K +'s) from 30MeV/u to 2 GeV/u. We finally discuss selection criteria for optimal probes with respect to the experimental determination of the nuclear equation of state at high densities.