Aneuploidy - the greatest genetic affliction of man - has its origin in manycellular processes. Its forerunner, chromosome non-disjunction, can result from malfunction of one of many processes involved in faithful segregation of chromosomes during mitosis or meiosis. Experts on chromosome biology, centromere, kinetochore, microtubules, spindle apparatus, biochemistry of mitosis and meiosis, developmental biology, and environmental mutagenesis discuss basic and applied aspects of how chromosomes malsegregate and how aneuplody originates. The most recent techniques employed to study chromosome segregation and also methods to determine aneugenic chemicals are described.