What are the different approaches to testing on agile development projects?

What are the different approaches to testing on agile development projects?
1. You want to test as early as you possibly can because the potential impact of a defect rises exponentially over time In fact, many agile developers prefer a test-first approach.
2. You want to test as often as possible, and more importantly, as effectively as possible, to increase the chance that you will find defects. Although this increases your costs in the short term, studies have shown that greater investment in testing reduces the total cost of ownership of a system due to improved quality.
3. You want to do just enough testing for your situation: Commercial banking software requires a greater investment in testing than membership administration software .
4. Pair testing, just like pair programming and modeling with others, is an exceptionally good idea.