Over the past 30 years, the methods available for the removal or improvement of acne scars and for the correction of wrinkle lines increased exponentially with the advent of new skin filler substances, improved techniques for elevating existing scars, and technology for abrading and resurfacing facial contours. Seemingly every month, a new and improved filling agent or laser becomes available, claiming to be the best. In the midst of these technological breakthroughs, the basic mechanisms remain the same. Three categories of techniques are presently available to improve scars: (1) scar removal and revision, (2) filling of depressed scars, and (3) contouring the surface of scars. 
Before and after images are below.
Numerous fillers are available. Each of these preparations has special properties, indications, and contraindications. Biodegradable fillers include bovine collagen (Zyderm/Zyplast, Resoplast), human collagen (Dermalogen), autologous fibroblasts (LAVIV, AlloDerm), hyaluronic acid (Hylaform gel, Restylane), calcium hydroxyapatite (Radiesse), and poly-L-lactic acid (PLLA). [3, 4] Nonbiodegradable fillers include silicone (off-label) and polymethylmethacrylate beads (Bellafill). 
Other procedures include surgical methods of replacing or implanting material into the dermis for scar elevation.
History of the Procedure
The goal for research and development in this field over the past 50 years has been the synthesis of the ideal filler substance for skin and soft-tissue augmentation. The ideal filling material has the following qualities:
Physiologic - Incorporates itself with the body's tissues
Simple procedure - Injectable
Risk-free - No complications or adverse effects
Permanent - Does not degrade with time
This ideal has not been met, but it has been the goal in creating all presently available materials.
Physiologic filling materials such as dermis, fascia, and fat have been used as autologous implants for many years, with specific surgical techniques used for each. These remain in use today but on the whole are cumbersome and complicated surgical operations with associated adverse effects and complications. Injectable filling materials included cutting oils, paraffin, and silicone, each with its own adverse effects and complications.
The first US Food and Drug Administration (FDA)–approved injectable filling agent, bovine collagen, was approved in 1982. Since then, several other agents have been approved. According to the American Society of Aesthetic Plastic Surgery Statistics, hyaluronic acid fillers ranked second in top nonsurgical cosmetic procedures in 2012 (1,423,705 procedures), behind botulinum toxin type A (3,257,913 procedures). 
Thirty years ago, scars were corrected using excision or dermabrasion techniques. With the advent of filling agents, additional options have become available for scar correction. This review discusses skin-filling agents available for scar correction.
Distensible cutaneous scars are best elevated with filling material. A distensible scar is defined as one that elevates to the surface when tension is placed on either side. This simple test allows the clinician to determine whether a particular scar is likely to respond to filling material. Fibrotic ice pick and fixed scars cannot be elevated because the scar tissue extends through to the subcutaneous tissue plane. Placing filling material in the dermis only elevates the surrounding skin and produces a donut effect, making the scar appear worse.
The ideal skin-filling agent is safe, physiologic, simple, and permanent. No known substance meets all of these criteria. Each of the available substances has adverse effects that detract from these properties, but all are the result of striving to perfect their qualities to fulfill these requirements. The list of injectable filling materials increases each month, with both synthetic and natural substances added. Other implant materials may be placed surgically.
Indications for collagen injection are acne scars, traumatic scars, photoaging, wrinkles, and gravitational rhytides.
Contraindications to this procedure include allergy to bovine collagen, allergy to lidocaine, pregnancy, or a collagen vascular disease such as dermatomyositis or polymyositis.