Every type of training, system, exercise, periodization, etc has its proponents and opponents.
Abdominal training is no different.
Some feel that only lifting with the big exercises is enough to train the abdomens. Others think that direct abdominal training is all that's needed to stimulate growth.
As per usual in the fitness industry, the answer lies somewhere in the middle.
I'm going to assume that you perform some or all of the big lifts such as benching, overhead pressing, squatting, pull-ups and/or deadlifts.
Personally, I have found that focusing solely on the big uns' while neglecting specific abdominal exercises is a recipe for disaster which has led to hip and back injuries in the past in my own training.
Ignoring direct trunk work will hinder the progress of your lifts, create an imbalance in the body and thus lead to potential injury.
Here are five abdominal exercises that I'd recommend everyone to introduce into their practices:
1) Ab Wheel: go for reps for around 3x6-8 to start.
2) Isometric Holds/Anti-Rotation (Plank, Side Bridge, Supine Bridge): hold for 30-60sec and create high tension throughout the body.
3) Mountain Climber Hold: keep the hips low, square and the pelvis tilted forward slightly. Hold for 20sec each side to begin with.
4) Eccentric Hanging Leg Raises (3-5 sec Tempo on the Way Down): go for reps here. Around 3 x 12-15 is a good start.