Astrophysicists have discovered two populations of black holes in the
universe: stellar-mass black holes with masses around ten
solar masses, and supermassive black holes with masses in the range
million to several billion solar masses. According to the general
theory of relativity, each black hole is completely described with
just two parameters: its mass and its spin. Measuring these two
parameters for individual black holes and relating the measured values
to specific observational manifestations of the holes is a major area
of research. The talk will describe some recent progress in this
exciting field.