  Phenomenologically, two classes of GRBs (long/soft vs. short/hard) are
identified based on their gamma-ray properties. The boundary between the two
classes is vague. Multi-wavelength observations lead to identification of two
types of GRB progenitor: one related to massive stars (Type II), and another
related to compact stars (Type I). Evidence suggests that the majority of long
GRBs belong to Type II, while at least the majority of nearby short GRBs belong
to Type I. Nonetheless, counter examples do exist. Both long-duration Type I
and short-duration Type II GRBs have been observed. In this talk, I review the
complications in GRB classification and efforts in diagnosing GRB progenitor
based on multiple observational criteria. In particular, I raise the caution to
readily accept that all short/hard GRBs detected by BATSE are due to compact
star mergers. Finally, I propose to introduce "amplitude" as the third
dimension (besides "duration" and "hardness") to quantify burst properties, and
point out that the "tip-of-iceberg" effect may introduce confusion in defining
the physical category of GRBs, especially for low-amplitude, high-redshift
GRBs.
