Incorrect usage of inhaler devices might have a major influence on the clinical effectiveness of the delivered drug. This issue is poorly addressed in management guidelines. This article presents the results of a systematic literature review of studies evaluating incorrect use of established dry powder inhalers (DPIs) by patients with asthma or chronic obstructive pulmonary disease (COPD). Overall, we found that between 4% and 94% of patients, depending on the type of inhaler and method of assessment, do not use their inhalers correctly. The most common errors made included failure to exhale before actuation, failure to breath-hold after inhalation, incorrect positioning of the inhaler, incorrect rotation sequence, and failure to execute a forceful and deep inhalation. Inefficient DPI technique may lead to insufficient drug delivery and hence to insufficient lung deposition. As many as 25% of patients have never received verbal inhaler technique instruction, and for those that do, the quality and duration of instruction is not adequate and not reinforced by follow-up checks. This review demonstrates that incorrect DPI technique with established DPIs is common among patients with asthma and COPD, and suggests that poor inhalation technique has detrimental consequences for clinical efficacy. Regular assessment and reinforcement of correct inhalation technique are considered by health professionals and caregivers to be an essential component of successful asthma management. Improvement of asthma and COPD management could be achieved by new DPIs that are easy to use correctly and are forgiving of poor inhalation technique, thus ensuring more successful drug delivery.