在概率论中，如果一个事件发生的概率为1，就说这个事件几乎必然发生（有时候简称a.s.）。这个概念与测度中的几乎处处是类似的。

在许多的基础概率试验中，几乎必然与必然是没有区别的，但是在一些与无穷相关的较为复杂的情况下，它们之间的区别是很重要的。例如，这个术语在无限时间、规律性特性或者无穷维度空间（比如函数空间）中往往需要加以讨论。基本的运用实例包括大数定律或者布朗运动的连续性。

几乎从不描述的是几乎必然的对立面，它是指一个事件发生的概率是零。

In probability theory, one says that an event happens almost surely (sometimes abbreviated as a.s.) if it happens with probability one. In other words, the set of possible exceptions may be non-empty, but it has probability zero. The concept is precisely the same as the concept of "almost everywhere" in measure theory. In probability experiments on a finite sample space there is no difference between almost surely and surely, but the distinction becomes important when the sample space is an infinite set (because an infinite set can have non-empty subsets of probability zero). Some examples of the use of this concept include the strong and uniform versions of the law of large numbers, and the continuity of the paths of Brownian motion.