Polyoxometalates (abbreviated as POMs), also known as early-transition metal-oxygen anion clusters, is a large and rapidly growing series of inorganic compounds. POMs can be generally divided into two subclasses: isopolyoxometalates(IPOMs) and heteropolyoxometalates (HPOMs). The former refers to a class of isopoly compounds which contain the skeletal transition metal cations and oxide ions of the same metal element. In contrast, the latter one, known as heteropoly compounds (HPCs) as well, contains both heteroatom and coordination atoms. This book reviews properties of POMs, their structure and also their synthesis.