Pectin and other pectic substances are complex polysaccharides, which contribute firmness and structure to plant tissues as a part of the middle lamella. The basic unit in pectic substances is galacturonan (α-D-galacturonic acid). Pectic substances are classified into two types; homogalacturonan and heterogalacturonan (rhamnogalacturonan). In homogalacturonan, the main polymer chain consists of α-D-galacturonate units linked by (1 → 4) glycosidic bonds, whereas in rhamnogalacturonan, the primary chain consist of (1→4) linked α-D-glacturonates and with about 24% L-rhamnose units that are β(1→2) and β(1→4) linked to D-galacturonate units (Whitaker, 1991). The side chains of rhamnogalacturonans usually consist of L-arabinose or D-galacturonic acid units. In plant tissues, about 6070% of the galacturonate units are esterified with methanol and occasionally with ethanol. Based on the degree of esterification, pectic substances are classified into protopectin, pectinic acid, pectin and polygalacturonic acid (Table 1). Molecular size, degree of esterification and weight distribution of polygalacturonic acid residues are important factors that contribute to heterogeneity in pectic substances. Relative molecular masses of pectic substances isolated from various sources such as citrus fruits, apple and plums, range from 25 to 350 kDa. Pectinases are a complex and diverse group of enzymes involved in the degradation of pectic substances. The diversity of forms of pectic substances in plant cells probably accounts for the existence of various forms of these enzymes. Pectinases are classified depending on their substrate and mode of enzymatic reaction (Fig. 1). Pectinases act as carbon recycling agents in nature by degrading pectic substances to saturated and unsaturated galacturonans, which are further catabolized (Table represented) to 5-keto-4-deoxy-uronate and finally to pyruvate and 3-phosphoglyceraldehyde (Vincent-Sealy et al., 1999). Pectinases from phytopathogenic fungi such as Aspergillus flavus, Fusarium oxysporum and Botrytis cinerea are also known to play a vital role in plant pathogenicity or virulence by degrading pectic compounds present in cell wall (Lang and Drenberg, 2000; DiPietro and Roncero, 1996; Ten Have et al., 1998). Pectinases, especially polygalacturonase, is known to play a major role in pectin breakdown during the final stages of fruit ripening (Sozzi- Quiroga and Fraschina, 1997; Chin et al., 1999). Polymethylgalacturonase (PMG), polygalacturonase (PG), pectin lyase (PL), polygalacturonate lyase (PGL) and pectinesterase (PME) are industrially important pectinases discussed in this chapter. © 2007 Springer.