The unique symmetry requirements of second harmonic generation (SHG) provide exquisite selectivity to chiral crystals, enabling independent quantitative modeling of the nucleation and crystal growth of active pharmaceutical ingredients (APIs) within amorphous solid dispersions (ASDs) during accelerated in situ stability testing, and in vitro dissolution testing. ASDs, in which an API is maintained in an amorphous state within a polymer matrix, are finding increasing use to address solubility limitations of small-molecule APIs. SHG microscopy yielded limits of detection for ritonavir crystals as low as 10 ppm, which is about two orders of magnitude lower than other methods currently available for crystallinity detection in ASDs. The quantitative capabilities of SHG analysis were substantially improved further while simultaneously dramatically reducing the total sample volume and storage burden through in situ analysis. Single particle tracking of crystal growth performed in situ enabled substantial improvements in the signal to noise ratio (SNR) for recovered crystal nucleation and growth rates by nonlinear optical microscopy. Upon dissolution, the presence of solubilizing additives in biorelevant media greatly affected the generation and stabilization of supersaturated solutions. SHG microscopy was found to enable the detection of crystals even in the highly turbid Ensure Plus® system. Analysis of the SHG micrographs clearly indicated that differences in the nucleation kinetics rather than growth rates dominated the overall trends in crystallinity. For weakly basic drugs, the fate of dissolution in fasted-state simulated intestinal fluid (FaSSIF, pH 6.5) varied with the ASDs drug loading, and was highly affected by the pre-exposure to the fasted-state simulated gastric fluid (FaSSGF, pH 1.6) medium, versus the dissolution in FaSSIF medium alone. The presence of crystals during the first stage of posaconazole ASDs dissolution in FaSSGF acted as nuclei for further crystallization in the later dissolution stage in FaSSIF. The results provide insights of better formulation prediction of poorly soluble drugs, as well as understanding origins of intraluminal absorption variability for such systems