It is a student's responsibility to choose a Ph.D. topic, with the help of the advisor. Sometimes, your advisor will have a specific research idea or will have a ready problem for you to work on. This work may directly lead to definining your Ph.D. topic. More frequently, however, you will have to find a topic on your own. Your advisor will be available to discuss it with you and offer you guidance, but will likely not initiate this process. What this means, is that you should take initiative and start searching for the topic, no later than the second year of the graduate school. Don't let yourself wake up in the fourth year of studies with a bunch of unrelated projects you worked on and without any idea of the suitable topic. See How to do research for guidance of how to choose a research topic. In addition to that, you need to choose a topic that:

• Will be significant enough so that you can write at least 3 papers on it
• Is doable in 2-3 year timeframe
• Your advisor and your committee members approve of

For a Ph.D. topic, more than for anything else, you need to make sure you like it enough to keep on working on it for 2-3 years. If you are not so sure, better spend some time finding out. Once you have chosen your topic and passed the Ph.D. qualifying exam, it is almost impossible to change it without changing the advisor and the school, and forfeiting all the time you already invested in this. So, if you don't like your topic, talk to your advisor as soon as possible. As I already mentioned here, you cannot do good research unless you are excited about the topic, which in Ph.D. thesis case translates to very, very long time spent doing research you don't like. If your advisor does not approve of your topic change, talk to them some more. Ultimately, you can also change the advisor. See How to work with your advisor and other faculty members for more information on this.

Ph.D. topics come roughly in three flavors. The first flavor is a topic that proposes a systematic solution to an important problem. You will typically design and implement this solution, then write several papers about different key principles of your solution. The second flavor is a topic that proposes a technique for a problem X, that also can be useful for related problems Y and Z. You will design and implement this technique, and test it for X, Y and Z, each test resulting in a paper. The third flavor is a topic that proposes several different techniques (say X, Y and Z) that address same or related problems. You will implement each of these techniques, test them on the given problem, and compare them against each other (and the related work).

Short point summary

• Take initiative in looking for your Ph.D. topic
• Make sure that both you and your advisor (and committee members) like the topic
• Ph.D. topic flavors (yeah, I'll have chocolate with mint;))