Biological processes are intrinsically dynamic. Although traditional methods provide valuable insights for the understanding of many biological phenomena, the possibility of measuring, quantifying, and localizing proteins within a cell, a tissue, and even an embryo has revolutionized our train of thoughts and has encouraged scientists to develop molecular tools for the assessment of protein or protein complex dynamics within their physiological context. These ongoing efforts rest on the emergence of biophotonic techniques and the continuous improvement of fluorescent probes, allowing precise and reliable measurements of dynamic cellular functions. The march of the "in vivo biochemistry" has begun, already yielding breathtaking results.