This thesis focuses on neutrophils in cancer, acute and chronic inflammation. The first part of this thesis describes how intravital microscopy (IVM) can be used in biomedical research to study dynamic processes at cellular and subcellular resolution in their environment and how different imaging windows including the abdominal imaging window (AIW), dermal imaging window (DIW) and cranial imaging window (CIW) can be implanted for facilitating the tracking of cells for different research purposes (chapter 2). To investigate the role of neutrophils in biopsy-induced tumor progression, we intravitally imaged the behavior of glioblastoma cells before and after biopsy via implanted CIW in neutrophil depleted mice (chapter 4). Different parameters, such as percentage and speed of migratory tumor cells, were measured to check if neutrophil play a role in biopsy-induced tumor progression. To confirm the hypothesis that neutrophil promote tumor progression, transwell assays and wound assays were also performed with human neutrophils in vitro. Although two- and three-dimensional in vitro studies of tumor cell lines are widely used and important for increasing our knowledge on tumor growth, behavior and metastasis formation, the complexity of in vivo microenvironment is not taken into consideration. Therefore, to better understand tumor cell behavior, we imaged tumors which developed from different common human and mouse breast tumor cell lines in living mice by intravital microscopy (chapter 3). Cell morphology, cell-cell interaction, polarity and motility of those tumor cell lines were measured. This data can serve as a resource to instruct researchers on the appearance and migratory behavior of the widely used breast tumor cell lines and warrants caution to the use of in vitro characterization of tumor invasiveness. The more traditional role of neutrophils is to phagocytose pathogens and clear damage upon acute inflammation. During inflammation, neutrophil phenotypes appear in the blood which are not present during homeostasis and they display different bacterial containment capacities. To explain the mechanism behind this difference, we first optimized neutrophil survival in a 3D in vitro model and then performed this assay with or without bioparticle phagocytosis and in the presence or absence of an MPO inhibitor (chapter 6). Apart from neutrophil heterogeneity in acute inflammation, neutrophil heterogeneity in cancer, COPD and trauma were also investigated and compared (chapter 5). Different maturation and activation markers were examined to further understand the similarities and differences of neutrophil populations under different conditions. Finally, in chapter 7, we summarize and discuss our main findings in the context of the aim of this thesis.