  A number of scenarios have been put forward to explain the origin of the
chemical anomalies (and resulting complex colour-magnitude diagrams) observed
in globular clusters (GCs), namely the AGB, Fast Rotating Massive Star, Very
Massive Star, and Early Disc Accretion scenarios. We compare the predictions of
these scenarios with a range of observations (including young massive clusters
(YMCs), chemical patterns, and GC population properties) and find that all
models are inconsistent with observations. In particular, YMCs do not show
evidence for multiple epochs of star-formation and appear to be gas free by an
age of ~3 Myr. Also, the chemical patterns displayed in GCs vary from one to
the next in such a way that cannot be reproduced by standard nucleosynthetic
yields. Finally, we show that the "mass budget problem" for the scenarios
cannot be solved by invoking heavy cluster mass loss (i.e. that clusters were
10-100 times more massive at birth) as this solution makes basic predictions
about the GC population that are inconsistent with observations. We conclude
that none of the proposed scenarios can explain the multiple population
phenomenon, hence alternative theories are needed.
