Histology is the science of the microscopic structure of cells, tissues and organs. It also helps us understand the relationship between structure and function. By examining a thin slice of bone tissue under a microscope, colorized with special staining techniques, you see that these seemingly simple bones are actually a complex microworld containing an array of structures with various different functions. In this article, we will introduce you to the microscopic world of histology. A cell is the smallest functional unit of an organism. All cells of the human body are eukaryotic, meaning that they are organized into two parts: nucleus and cytoplasm. The cytoplasm contains specialized subunits called organelles which work like ‘little organs’. Organelles can be membranous (mitochondria, Golgi apparatus, endoplasmic reticulum) or non-membranous (ribosomes, nucleolus, centrioles). Histology has always played an important role in diagnosis and subsequent treatment options. The advent of molecular testing can evaluate a patient’s overall outcome (prognosis) or identify whether a patient qualifies for a specific therapy (predictive), making it imperative to produce the highest quality tissues. Standardization of the fixation and processing of tissue has become a focus due to the increased use of companion diagnostics. Pre-analytic, analytic and post-analytic variables are addressed in the published guidelines for tests linked to companion drug treatments. The requirements given ensure accurate demonstration of prognostic markers.