Cryogenic sensors are one of the most powerful and promising instruments for future
astronomical applications. Transition Edge Sensors (TESs) are well-advanced cryogenic
sensors among others. TESs are worked at 100 mK and provide a good temperature
sensitivity which enables to provide a theoretical spectral resolution of E/dE~6000 at
X-ray regime (6 keV).  To archive its theoretical limit, the understanding of the
physics of supercondactivities plays an important role.

In parallel to detector physics, the signal multiplexing technology is also one of the
key aspects for space application because of limited power availability in the orbit. In
SRON, we are developing signal readout technology based on frequency domain multiplexing
(FDM). The remarkable development of FDM has been made over the last 10 years, which
pushes the matureness of FDM technology high enough to use space-based projects.  In
this contribution, I will introduce the recent progress of TESs X-ray microcalorimeter
and the FDM technology.