Tissue imaging lies in the heart of patient care. In fact, cyto-histopathological diagnostic imaging, through the microscopic examination of cellular and tissue samples, is considered an essential component of clinical care as a diagnostic and pathognomonic aid. Traditional pathology is inherently invasive as it de facto requires the removal of the biological sample from the human body. Moreover, the necessary incorporation of processing steps, including those with fixatives and staining agents, not only delays the diagnostic outcome, but also introduces biochemical artifacts such as loss of tissue components, tissue shrinkage and constituent cross-linking. It is thus evident that standard cyto-histopathological evaluation, despite its undisputable contributions to patient care, is unable to study the dynamic nature of physiological and pathophysiological processes in an unperturbed, real-time, in situ manner. Novel optical imaging modalities offer the unique opportunity to obviate the need for tissue excision and enable non-destructive, in vivo morpho-functional analysis. Nonlinear optical imaging (NLO) embodies an assortment of powerful microscopy techniques that generate high-contrast, high-resolution, three-dimensional imaging by exploiting nonlinear, high-order light-matter processes. The application potential of NLO multi-photon microscopy (MPM), through two-photon excited fluorescence (TPEF), fluorescence Lifetime Microscopy (FLIM) and second harmonic generation (SHG) imaging, represents the emphasis of this dissertation. Numerous applications are explored in the context of characterizing intracellular or extracellular morpho-functional changes for diagnostic, monitoring and treatment related biomedical applications in a noninvasive, label-free, unperturbed manner in situ. These cover complex and heterogeneous spatiotemporal biological systems in vitro, ex vivo and in vivo and ultimately lead to the diagnostic evaluation of all levels of cancerous progression (pre-cancer, invading cancer, metastasis) in human tissues. Ultimately this dissertation aims to highlight that the knowledge gained from elucidating the morpho-functional interplay of tissue homeostasis and disease progression through noninvasive NLO multi-photon imaging, may play a transformative role in the pathophysiological mechanistic understanding of tissue biology and in the development of improved diagnostic systems and therapeutic regimens in a broad range of biomedical and clinical applications.