The practical application of nanoparticles requires transitioning from well controlled experimental settings to highly variable "real-life" conditions. Understanding the resulting changes in the behavior and stability of nanoparticles is therefore of paramount importance. This thesis discusses the development and practical applications of tools to monitor the behavior of nanoparticles in real-time using intensity correlation spectroscopy techniques. I show how-correlation spectroscopy can be adapted to nanoparticle systems; and provide particular parameters and settings especially vital for heterogeneous systems. Oftentimes nanoparticles have to be labeled to be detected, which can complicate the system of study and can introduce systematic errors into the analysis. Intensity correlation spectroscopy was tested on dye-labeled magnetite nanocrystals. The fluorescence correlation spectroscopy results were surprisingly biased towards a low concentration of aggregates. Scattering and absorption cross-sections of gold nanoparticles are greatly enhanced near the plasmon resonance wavelength, providing strong intrinsic signals for directly visualizing nanoparticles. I show here how scattering and absorption scale with nanoparticle size; and how size heterogeneity within nanoparticle samples translates into the detected signals. One-photon luminescence of gold nanoparticles, an often neglected signal, was also considered. A comparison between one-photon luminescence and scattering correlation spectroscopy revealed that the former has a much smaller bias towards aggregates and therefore is advantageous in systems prone to aggregation. Overall, the work presented here describes the tools and methods that were developed towards better understanding of nanoparticle behavior in a liquid medium where they are to be employed for environmental and biological applications.