The Cincinnati Bengals will kickoff their first voluntary workout on April 15, two weeks before the 2013 NFL draft. According to league rules mandated by the Collective Bargaining Agreement, there are three phases of offseason workouts.
Phase One restricts most activities to strength and conditioning and physical rehabilitation only. It's so restricted that the language in Article 21, Section 2(i) of the CBA reads that, "only full-time or part-time strength and conditioning coaches, who have no other coaching responsibilities with the Club, shall be allowed on the field; no other coaches shall be allowed on the field or to otherwise participate in or observe activities."
The second phase is a three-week program (dates currently unknown) in which all coaches are allowed on the field, observing and supervising individual workouts, drills and instruction. However teams are only allowed to conduct "perfect play" drills; only the offense or the defense (no offense vs. defense). Additionally no live contact is permitted, nor offense vs. defensive drills (offensive linemen vs. defensive linemen, receivers vs. secondary, etc.). The same is applied to special teams. In fact players aren't even allowed to wear helmets during Phase Two.
Phase Three is ten organized team practice activities (aka, OTA) later this summer. According to the CBA, three practices may take place in the first two weeks of Phase Three with third or fourth week allowing a maximum of four. Much like phase two, there are contact restrictions. No live contact is permitted, nor any one-on-one offensive versus defensive drills. Teams may require players to wear helmets, but no shells. The fourth week (or third, depending on how the OTAs are distributed, will be the team's mandatory minicamp which takes place on June 11-13.