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This thesis is motivated by the need to improve our understanding of the aerosol indirect effect. The activation of aerosol into cloud droplets has been extensively studied, using a comprehensive numerical cloud droplet activation model. Using this model, the effect of water vapor mass transfer limitations on the cloud droplet activation process was first studied; it was found that mass transfer limitations are important for activation under polluted conditions. The potential effect of (currently unresolved) ``chemical effects' on cloud droplet number (e.g., the presence soluble gases and surface active species) was also assessed. It was seen that small changes in aerosol and gas-phase composition can have a strong effect on cloud droplet number, and should be included in future estimates of the aerosol indirect effect.

A comprehensive aerosol activation parameterization was developed for use in a first-principle assessment of the aerosol indirect effect. This new parameterization introduces the concept of ``population splitting,' in which the droplets are separated into two populations, each with its own growth characteristics. The effect of water vapor mass transfer limitations is explicitly introduced. The parameterization allows for treatment of chemical effects. The new parameterization shows excellent and robust agreement with a detailed numerical parcel model.

Previously unidentified mechanisms of aerosol-cloud interactions were also explored. Aerosol, when it contains black carbon, can absorb light and heat the droplet enough to affect its activation behavior. This can affect cloud properties in numerous and counterintuitive ways; black carbon heating can dissipate clouds, but may also increase cloud lifetime (and lead to a climatic cooling) by decreasing the probability of drizzle formation.

Finally, the performance of instruments used for measuring the concentration of cloud condensation nuclei (CCN) was assessed. Each design exhibits different limitations and sources of uncertainty, but all show decreased ability to measure CCN of low critical supersaturation ($<$ 0.1\%). The performance of the instrumentation can be very sensitive to the operating conditions. Therefore, an in-depth theoretical understanding of the instrumentation is necessary; otherwise, CCN measurements may be subject to considerable uncertainty.

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