The aerosol effects on the climate system are roughly divided into three categories: direct, indirect, and semi-direct effects. Observations from satellites and ground with remote sensing and numerical models have been developed to understand and estimate aerosol effects on a global scale. In the latest assessment report of the Intergovernmental Panel on Climate Change (IPCC) , however, there are still large uncertainties in their radiative forcings in comparison with the estimation of long-lived greenhouse gases. To reduce the uncertainties, we have to study the three-dimensional aerosol distributions and the cloud-aerosol interaction more accurately. It is important to observe aerosol vertical profiles with lidar, one of the active sensors, to understand the three-dimensional aerosol distributions as well as to continue observations with passive sensors. The data assimilation, which harmonizes numerical models with observations, is also an effective method to reduce the uncertainties. A cloud resolving model coupled with an aerosol transport model is a useful tool to better understand the cloud-aerosol interaction. Efforts to analyze the aerosol climate effects quantitatively will result in more reliable projection of the future climate change and elucidating climate system.