Whether you go Roth or traditional, you need to read up on the different funds within the TSP that you can invest in. By default, your contributions are assigned to the G fund, which is invested in bonds. The G fund is guaranteed to never lose money, but it doesn't have a very good rate of return and isn't a good long-term investment. I'm invested 50/50 in the C and S funds, which are both stock funds. As I get closer to retirement, I'll start shifting money into one of the less-volatile funds. They also offer "Lifestyle" funds geared toward your targeted retirement date. These funds consist of a mix of their other funds and the TSP will shift the holdings toward the more conservative funds as it gets closer to the target end date so that you don't have to worry about doing it yourself.