This review describes key directions in the development of different probes based on complex compounds of lanthanides for in vitro and in vivo researches. The role of microsecond fluorescence of lanthanides for overcoming problems of background fluorescence is considered. The basic classes of synthetic and genetically encoded complex compounds of lanthanides are summarized. Main principles of designing lanthanide-based molecular sensors, including FRET sensors based on lanthanides and colored fluorescent proteins are described. Their applications in bioanalytical research and cellular bioimaging are described. The main principles of cellular bioimaging using lanthanides are formulated, questions of their delivery into cells are considered, and the problem of their potential toxicity for living organisms is discussed. A technique using multiphoton excitation of lanthanides is described.