  This talk focuses on the statistical properties of X-ray selected BL Lacertae
objects (XBLs) whose catalogue has been compiled. It consists of 312 sources
from different X-ray surveys, unambiguously identified to mid-2010. Results of
the statistical research of different observational quantities (redshift,
muliwavelength luminosities, host/nucleus absolute Magnitudes, central black
hole masses, synchrotron peak frequencies, broadband spectral indices) are also
provided and existence of the correlation between them is proved. Overall flux
variability shows an increasing trend towards greater frequencies. XBL are
found to be much less active in point of intra-night optical variability
compared to radio-selected BL Lacs (RBLs). A separate list of 106 XBL
candidates is also created including the same characteristics for each source
as in the case of XBL catalogue.
