This chapter presents an overview of our current understanding of the indirect effects of aerosol particles on cloud microphysical and radiative properties, focusing both on in situ and remote-sensing measurements that have led to an enhanced understanding of human impact on climate. Due to the difficulty of separating the influence of cloud condensation nuclei increases on cloud droplet concentration and hence cloud radiative properties, we concentrate our attention on stratocumulus clouds in the remote marine environment. We begin this review by defining cloud susceptibility and hence why some clouds are more likely to respond to enhanced aerosol injections than other clouds. We discuss in some detail a selected number of observations that demonstrate indirect effects and cloud susceptibility, including small-scale observations of clouds modified by aerosol emissions from ships (\ship tracks\") and large-scale observations of marine stratocumulus clouds in both maritime and continental air masses.

Finally, we will clarify the significant and complimentary roles that in situ microphysical and radiation instrumentation play in quantifying the indirect effects of aerosol on cloudsas well as the added value that arises from coordinating these measurements with remote-sensing observations from aircraft or satellite.