  The origin and nature of Ultra-High Energy Cosmic Rays (UHECRs) remain
unsolved in contemporary astroparticle physics. To give an answer to these
questions is rather challenging because of the extremely low flux of a few per
km$^2$ per century at extreme energies (i.e. E $>$ 5$\times$10$^{19}$ eV). The
central objective of the JEM-EUSO program, Joint Experiment Missions for
Extreme Universe Space Observatory, is the realisation of an ambitious
space-based mission devoted to UHECR science. A super-wide-field telescope will
look down from space onto the night sky to detect UV photons emitted from air
showers generated by UHECRs in the atmosphere. The JEM-EUSO program includes
several missions from ground (EUSO-TA), from stratospheric balloons
(EUSO-Balloon, EUSO-SPB1, EUSO-SPB2), and from space (TUS, Mini-EUSO) employing
fluorescence detectors to demonstrate the UHECR observation from space and
prepare the large size missions K-EUSO and POEMMA. We review the scientifical
objectives associated with the developing projects of the JEM-EUSO program and
the technological achievements allowing them.
