This chapter covers definitions of suicidal behaviours and how they vary over time, reflecting predominant philosophies and schools of thought. The limitations in the quality of information about suicide mortality, as a common feature affecting the whole vital registration system, are discussed. The smaller the coverage a country receives, the greater the probability of distortions, which adds to any previous distortions already flawing the data. It should be strongly emphasized that these shortcomings affect the system as a whole, and hence all causes of death. However, suicidologists seem to be much more punctilious about under-reporting of suicide, and the essential unreliability of this information, than experts dealing with mortality from other causes. Coordinated efforts should be made to strengthen those systems, paying attention to the specificity of sociocultural factors’ influence on defining, recording, and reporting suicide as a cause of death.