Since reliability engineering is focused on the survivability or absence of failures, it is more concerned about failures, understanding their causes and defining remedies. Therefore, it is essential to have a clear understanding of the term "failure". A failure is the partial or total loss or change (degradation) in those properties of a system such that its functioning is impeded or stopped. Some components have well defined failures; others do not: For example: Switches are either operational or non-operational. Such devices are known as two state devices. On the other hand, devices like resistors can have a range of operating conditions (say R + r). They fail only when the resistance value goes out of this range. Such devices can be termed as multistate devices. Partial failures (degraded performance) are other types of failures. Failures can also be classified based on failure mechanisms and failure modes. In many cases, it can be seen that some failures are subjective while others are objective in nature. Identifying failures and failure modes is the first step towards achieving high reliability.