Crystal engineering is a broad area of research that focuses on methods of designing and/or optimizing materials for diverse applications in fields spanning from energy to medicine. The ability to selectively control crystallization to achieve desired material properties requires detailed understandings of the thermodynamic and kinetic factors regulating crystal nucleation and growth. Combining this fundamental knowledge with innovative approaches to tailor crystal size, structure, and morphology can lead to the production of materials with superior properties beyond what is achievable by conventional routes. In this talk I will discuss two general mechanisms of crystal growth: (1) classical pathways involving 2-dimensional layer nucleation and advancement on crystal surfaces through monomer addition; and (2) nonclassical pathways, termed crystallization by particle attachment (CPA), involving the formation of metastable precursors that play a direct role in crystal nucleation and growth. Our group uses techniques such as atomic force microscopy (AFM) to investigate crystallization in situ under solvothermal conditions. We have developed a unique AFM system capable of capturing time-resolved dynamics of surface growth, thus opening new routes to probe complex pathways of crystallization. We also design “modifiers” to control crystal properties such as size and morphology. Modifiers are molecules or macromolecules that interact with specific surfaces of crystals and regulate anisotropic growth rates. In this talk, I will show how we use growth modifiers to control crystallization in two distinctly different, yet fundamentally similar, applications. In the first part of my talk I will discuss our work on the development of therapeutic drugs for crystals implicated in two pathological diseases: kidney stones (calcium oxalate monohydrate) and malaria (hematin). In the second part of my talk, I will discuss how we are using modifiers as a bio-inspired approach to tailor the properties of zeolites, which are microporous aluminosilicates commercially used in catalysis, adsorption, and ion-exchange processes. Topics that will addressed include the broader challenges of synthesizing zeolites, progress towards elucidating their complex mechanism(s) of growth, and extensive effort to develop commercially-viable approaches to tailor their physicochemical properties.