There are two kinds of positive cash flows: pre-tax and after-tax. A pre-tax positive cash flow occurs when income received is greater than expenses incurred. This sort of situation is difficult to find, but they are usually a strong and safe investment. An after-tax positive cash flow may have expenses that outweigh collected income, but various tax breaks allow for a positive cash flow. This is more common, but it is generally not as strong or safe as a pre-tax positive cash flow. Regardless of what kind of real estate you choose to invest in, timely collections from your tenants is absolutely necessary. A positive cash flow -- whether it be pre-tax or after-tax -- requires rental income. Be sure to find quality tenants; a thorough credit and employment check is probably a good idea.