  We have summarized the current understanding and recently obtained findings
about WZ Sge-type dwarf novae. We also reviewed the historical development of
the understanding of these objects, provided the modern criteria, and reviewed
the past research in relation to superhumps, early superhumps and the outburst
mechanism. We regard that the presence of early superhumps (reflecting the 2:1
resonance) and long or multiple rebrightenings are the best distinguishing
properties of WZ Sge-type dwarf novae. We provided the updated list of nearly
100 WZ Sge-type dwarf novae mainly based on the data obtained by the VSNET
Collaboration up to Kato et al. (2015, arXiv/1507.05610) and discussed the
statistics. We could detect early superhumps with amplitude larger than 0.02
mag in 63% of the studied WZ Sge-type dwarf novae, which makes early superhumps
a useful distinguishing feature for WZ Sge-type dwarf novae. Theoretical light
curves of early superhumps generally appear to reproduce the existence of many
low-amplitude objects, supporting the geometrical origin of early superhumps.
Using the recently developed method of measuring mass ratios using developing
phase of superhumps (stage A superhumps), we showed that there is a linear
relation between the period variation of superhumps and the mass ratio in WZ
Sge-type objects. By using this relation, we were able to draw an evolutionary
picture of a large number of WZ Sge-type and identified the type of outburst to
be an evolutionary sequence: type C->D->A->B->E, with some outliers for type-B
objects. The duration of stage A (evolutionary phase) of superhumps is also
well correlated with the estimated mass ratios. By using mass ratios from stage
A superhumps and durarion of stage A, we have been able to identify best
candidates for period bouncers.
