 Black holes are regions of spacetime from which nothing, not even light, can escape. A typical black hole is the result of the gravitational force becoming so strong that one would have to travel faster than light to escape its pull. Such black holes generically contain a spacetime singularity at their center; thus we cannot fully understand a black hole without also understanding the nature of singularities. Black holes, however, raise several additional conceptual problems and questions on their own. When quantum effects are taken into account, black holes, although they are nothing more than regions of spacetime, appear to become thermodynamical entities, with a temperature and an entropy. This seems to point to a deep and hitherto unsuspected connection among our three most fundamental theories, general relativity, quantum field theory and thermodynamics. It is far from clear, however, what it may mean to attribute thermodynamical properties to black holes. At the same time, some of these thermodynamical properties of black holes now seem amenable to direct testing in terrestrial laboratories by observing the behavior of “analogue” systems composed of ordinary material. This all raises problems about inter-theory relations, in particular about relations between the “same” quantity as it appears in different theories. It also bears on the meaning and status of the Second Law of thermodynamics, with possible implications for characterizing a cosmological arrow of time. 