  We present Swift observations of the black hole X-ray transient, GRO
J1655-40, during the recent outburst. With its multiwavelength capabilities and
flexible scheduling, Swift is extremely well-suited for monitoring the spectral
evolution of such an event. GRO J1655-40 was observed on 20 occasions and data
were obtained by all instruments for the majority of epochs. X-ray spectroscopy
revealed spectral shapes consistent with the ``canonical'' low/hard, high/soft
and very high states at various epochs. The soft X-ray source (0.3-10 keV) rose
from quiescence and entered the low/hard state, when an iron emission line was
detected. The soft X-ray source then softened and decayed, before beginning a
slow rebrightening and then spending $\sim 3$ weeks in the very high state. The
hard X-rays (14-150 keV) behaved similarly but their peaks preceded those of
the soft X-rays by up to a few days; in addition, the average hard X-ray flux
remained approximately constant during the slow soft X-ray rebrightening,
increasing suddenly as the source entered the very high state. These
observations indicate (and confirm previous suggestions) that the low/hard
state is key to improving our understanding of the outburst trigger and
mechanism. The optical/ultraviolet lightcurve behaved very differently from
that of the X-rays; this might suggest that the soft X-ray lightcurve is
actually a composite of the two known spectral components, one gradually
increasing with the optical/ultraviolet emission (accretion disc) and the other
following the behaviour of the hard X-rays (jet and/or corona).
