证明书种类很多，有工作经历证明、工作经验证明、病情证明、留学生经济担保书、学业成绩证明书等等，是用来证明一个人的身份、学历、婚姻状况、身体情况等或某一件事情的真实情况。证明书的写法通常也采用一般信件格式，但多省掉收信人的姓名、地址和结束用语。称呼多用“To Whom It May Concern”意即“有关负责人”，但此项也可省略。写证明书要求言简意赅。
例一：医院证明
Doctor's Certificate
June 18, 2000
This is to certify that the patient, Mr. Tomas, male, aged 41, was admitted into our hospital on June 9, 2000, suffering from acute appendicitis. After an immediate operation and ten days of treatment, he has completely recovered and will be discharged on June 19, 2000. It is suggested that he rest for one week at home before resuming his work.
Jack Hopkins
Surgeon-in-charge
医生证明书
兹证明病人托
主治医生：杰克。霍普金斯
2000年6月18日
