Synovial fluid (SF) accumulates in the joint cavity in different conditions; this review outlines the data from those analyses that help in their differential and definitive diagnosis. The gross appearance of the fluid can provide a quick bedside orientation with regard to the amount of inflammation present in the joint: totally transparent SF originates in non-inflammatory conditions--of which osteoarthritis is the most common--and the amount of turbidity grossly relates to the amount of inflammation. Most turbid to purulent fluids usually come from infected joints, but exceptions are not uncommon. The white cell count offers quantitative information, but the boundaries between non-inflammatory and inflammatory SF and between this and septic fluid are very hazy and figures have to be interpreted in the clinical setting. Detection and identification of monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD) crystals allow a precise diagnosis of gout and CPPD crystal-related arthropathy. Only one in five CPPD crystals have sufficient birefringence for easy detection and they are easily missed if searched for only using a polarised microscope. Instructions for beginners are given. Proper microbiological studies of the SF is the key to the diagnosis of infectious conditions.