The use of organometal trihalide perovskites (OTPs) in perovskite solar cells (PSCs) is revolutionizing the field of photovoltaics, which is being led by advances in solution processing of OTP thin films. First, we look at fundamental phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution involved in the solution-processing of OTP thin films for PSCs from a materials-science perspective. Established scientific principles that govern some of these phenomena are invoked in the context of specific literature examples of solution-processed OTP thin films. Second, the nature and the unique characteristics of OTP thin-film microstructures themselves are discussed from a materials-science perspective. Finally, we discuss the challenges and opportunities in the characterization of OTP thin films for not only gaining a deep understanding of defects and microstructures but also elucidating classical and nonclassical phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution in these films. The overall goal is to have deterministic control over the solution-processing of tailored OTP thin films with desired morphologies and microstructures.