The mechanism of formation of polymer molecules in the gas phase is difficult to study because the involatile polymers tend to condense out of that phase. However, new techniques, involving the use of cloud chambers, have enabled workers to use the nucleation of liquid drops in supersaturated monomer vapors to detect single polymer molecules and therefore to work with so few simultaneously growing polymers that aggregation and condensation are avoided. Chain polymerization in which the chain carriers are either radicals or ions can therefore be studied in the vapor. Furthermore, the ability to work with such small concentrations of growing polymeric radicals, for example, makes it possible to avoid encounters between them that lead to recombination and the formation of "dead" polymers that are incapable of further growth. Many aspects of gas-phase polymerization can be studied including, besides radical and ion chains, ring-opening polymerization, initiation, radiation-induced polymerization, and especially "ultraslow" chemistry.