透析是利用半透膜将小分子和大分子分离的一种技术。

在医学中，透析被用来代替因为肾衰竭而丧失功能的肾，俗称「洗肾」。透析可以被用来救助突然的暂时丧失其肾功能的病人（急性肾衰竭），也可以被用来救助永久性丧失肾功能的患者（慢性肾脏病）。

一般来讲，透析有血液透析和腹膜透析两种。

在很多情况下，血液透析也可以用于对病人的抢救，如摇头丸引起横纹肌溶解症等。 因为血液透析和其他药物的出现战场上由于战伤引起的急性肾衰竭的死亡率已由超过90％大幅下降。

对于腹膜完整且神智清醒能自我操作腹膜透析器材的病人，腹膜透将给予病人更大的自由。因为腹膜透析的不间断性，在感染控制得当的情况下它的并发症较少。

In medicine, dialysis (from Greek διάλυσις, diàlysis, meaning dissolution, διά, dià, meaning through, and λύσις, lỳsis, meaning loosening or splitting) is a process for removing waste and excess water from the blood and is used primarily as an artificial replacement for lost kidney function in people with kidney failure. Dialysis may be used for those with an acute disturbance in kidney function (acute kidney injury, previously acute renal failure) or progressive but chronically worsening kidney function—a state known as chronic kidney disease stage 5 (previously chronic renal failure or end-stage renal disease). The latter form may develop over months or years, but in contrast to acute kidney injury is not usually reversible and dialysis is regarded as a "holding measure" until a kidney transplant can be performed or sometimes as the only supportive measure in those for whom a transplant would be inappropriate.