The rudiments of a self-consistent theory of intraseasonal and interannual variability for the tropical ocean-atmosphere system are presented. In this paper we study some basic properties of low frequency phenomena in the tropical atmosphere and coupled ocean-atmosphere system and attempt to seek a unified dynamical framework in which the mechanisms of intraseasonal oscillations and ENSO can be studied and understood. Specific physical processes are identified and their roles in leading to intraseasonal oscillations and ENSO are elucidated both separately and collectively for a simple shallow-water system. A common thread linking the two phenomena is in the vital role played by moist processes in the atmosphere and how they can be affected by mean sea surface temperature (SST) and interactive SST anomaly. The importance of including a time-dependent, moist atmosphere in studies of the coupled atmosphere-ocean system is emphasized. For intraseasonal oscillations, results show that the slow eastward propagation in the atmosphere is due to condensation-convergence feedback leading to a reduction of the effective static stability of the troposphere. The presence of evaporation-wind feedback is important in further modifying this basic mechanism. The slow motions are favored over a warm, moist atmosphere directly as a result of increased saturated moisture due to higher SST. In the coupled moist atmosphere-ocean system, two unstable modes i.e., an advective mode and an upwelling mode, are identified. The advective mode is due to the destabilization of atmospheric waves by air-sea interaction through east-west SST advection. In this mode, the region of maximum convection lies about 10° west of the maximum SST anomaly. The structure and propagation of the atmospheric and oceanic anomalies in the advective mode are reminiscent of the growth phase of the 1982–83 ENSO. On the other hand, the upwelling mode is due to the destabilization of oceanic Kelvin waves by air-sea interaction through oceanic upwelling. This mode has no east-west displacement between atmospheric convection and SST anomaly and for low SST (typical below 260°C), corresponds to an unstable mode in the ‘dry’ atmosphere found in previous studies. Most importantly, both modes show a dramatic increase in growth rate through air-sea coupling as the mean SST increases from 27° to 28°C. The above results clearly demonstrate the importance of total SST as well as the SST anomaly in playing different but crucial roles in leading to unstable air-sea interaction. A necessary condition for instability in the coupled system is also derived. This condition provides a test from which the proposed theory may be partially validated from observations.