  The physical and chemical properties of prestellar cores, especially massive
ones, are still far from being well understood due to the lack of a large
sample. The low dust temperature ($<$14 K) of Planck cold clumps makes them
promising candidates for prestellar objects or for sources at the very initial
stages of protostellar collapse. We have been conducting a series of
observations toward Planck cold clumps (PCCs) with ground-based radio
telescopes. In general, when compared with other star forming samples (e.g.
infrared dark clouds), PCCs are more quiescent, suggesting that most of them
may be in the earliest phase of star formation. However, some PCCs are
associated with protostars and molecular outflows, indicating that not all PCCs
are in a prestellar phase. We have identified hundreds of starless dense clumps
from the mapping survey with the Purple Mountain Observatory (PMO) 13.7-m
telescope. Follow-up observations suggest that these dense clumps are ideal
targets to search for prestellar objects.
