Abstract : The problem of laser cooling of a single ion or atom in a harmonic trap was considered. A simple theory of sideband cooling has been developed. In the limit that the particle's secular motion can be treated semiclassically, the theory allows the calculation of a logarithmic cooling rate either numerically or, for two special cases, analytically. The theory could be used to optimize the parameters of a cooling experiment. The spectroscopy of a single atom in the ladder configuration has been treated theoretically. A dressed atom approach was used to provide qualitative information about the system.