Crystal diseases are among the older conditions to affLict human beings. Gout is caused by elevations in serum urate concentrations above its saturation point of 6.8mgdL-1 (hyperuricemia), such subsequent precipitation in tissues in monosodium urate (MSU) form and generation of an inflammatory reaction. Hyperuricemia develops as a consequence of excessive purine production or defective purine excretion in the kidneys. In addition, unLike most other mammals, humans lost the abiLity to degrade uric acid into allantoin, for unknown reasons, in the Miocene period. The inflammatory reaction to MSU crystals is driven by the inflammasome complex with generation of interleukin-1β. A better understanding of the mechanisms of hyperuricemia and gouty inflammation has led to advances in the treatment of gout. Calcium pyrophosphate (CPP) deposition disease occurs after the articular formation of CPP crystals. An imbalance between the tissue pyrophosphate concentration and phosphatase activity in the articular matrix around the chondrocyte leads to CPP crystal formation. Basic calcium phosphate arthropathy is a highly inflammatory process of periarticular structures generated by basic calcium phosphate crystals. Polarized Light microscopy is the preferred method for diagnosis of the most common crystal arthropathies.