The onset and development of flow in a thick horizontal layer subject to a nearconstant flux heating from below has been studied experimentally. The overall heatflux- based Rayleigh number, Ra * ranges from 2 × 105 to 7 × 1010. Flow visualization shows the growth and breakdown of a conduction layer adjacent to the heated surface. Convection is characterized by the release of warm meandering plumes and thermals from a boundary layer. The planform of convection at the heated surface begins with a pattern of small spots suggestive of Benard cells. Some of these cells expand, forming a larger cell pattern. This continues until a quasi-steady state is reached in which the former cell boundaries form a slowly moving pattern of warm lines on the heated surface. The lines are believed to be the source of the plumes and thermals. Quantitatively, the onset of convection occurs at a constant (critical) Rayleigh number based on the conduction layer thickness, Ra6. Based on the first observation of fluid motion, this critical Rayleigh number is approximately 1300. Based on the heated surface temperature the critical Rayleigh number is 2700. The nondimensional wavenumber associated with the observed instabilities at the onset of convection is about 2.2.