〜以上(は)
since, as long as (it follows that)
Plain form + 以上(は) (な-adj, Noun + である)
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以上は gives a premise that leads to an obligation or a firm conclusion: 約束した以上、守るべきだ, "since you promised, you should keep it". The は is often left off.
It covers both "now that" (a decision already made) and "as long as" (an ongoing state): 学生である以上は、勉強が第一だ, "as long as you're a student, studying comes first".
The second half is a judgement, duty or decision: べきだ, なければならない, わけにはいかない, できない. It sits between からには (a bit more personal) and 上は (the most formal).
After nouns and な-adjectives, use である: 学生である以上. Don't confuse it with 以上 as a noun ("more than", or "that's all" at the end of a speech).
Examples
約束した以上、守るべきだ。
Since you promised, you should keep your word.
学生である以上は、勉強が第一だ。
As long as you're a student, studying comes first.
やると言った以上、途中でやめるわけにはいかない。
Since I said I'd do it, I can't quit halfway.
日本に住む以上、日本の法律に従わなければならない。
As long as you live in Japan, you have to obey Japanese law.
証拠がない以上、彼を疑うことはできない。
As long as there's no evidence, we can't suspect him.