A Remote Pilot Certificate (a.k.a. Part 107 license) is mandatory if you plan on flying your drone for all non-recreational purposes. Part 107 rules are applicable for all drones weighing less than 55 pounds and used for non-hobby purposes. If you are flying a large drone weighing more than 55 pounds, a Part 107 license will not suffice. You will have to apply for an exemption under the Special Authority for Certain Unmanned Systems (which has replaced Section 333). If you are planning only flying for recreation and/or hobby purposes, you do not need a Part 107 license. But soon, you will need to pass an online safety exam.  While the exam doesn't exist yet, recreational pilots now have to apply for LAANC airspace authorization to fly in controlled airspace.And if you are a manned pilot with a Part 61 private pilot license, the process is much simpler. But you still need to get your 107 Remote Pilot Certificate. There are a few conditions to getting your 107 under your Part 61 license. These are the conditions for commercially flying drones with a Part 61 license - Hold a pilot certificate issued under 14 CFR part 61; and Have completed a flight review within the previous 24 months; and You must take a small UAS online training course provided by the FAA; and Keep you Remote Pilot Certificate card "easily accessible by the remote pilot" you when you're out flying.