Three-dimensional echocardiography (3DE) represents a major innovation in cardiovascular imaging. Advancements in computer and transducer technologies have made this modality an important diagnostic tool in the cardiology clinic. Besides permitting novel views and comprehensive presentation of cardiac structures from any spatial point of view, from a single 3D acquisition, it is also useful in providing fast accurate, and reliable measurements of chamber size and function, including the quantification of left ventricular (LV) mechanical dyssynchrony to guide patient selection for cardiac resynchronization therapy (CRT). Three-dimensional speckle tracking echocardiography (3DSTE) is a next step forward toward a more comprehensive evaluation of the LV, including principal chamber indices such as LV volumes and ejection fraction (EF), but also parameters directly assessing global and regional myocardial function such as longitudinal, radial and circumferential strain. Moreover, also new parameters are available such as area strain and principal strain. To continue to improve the clinical applicability of 3DE imaging to become part of the routine clinical practice, further refinements in automatic quantitative analysis software that will enable fast online measurements that are accurate, reliable, and vendor-independent are required. The aim of the present work in this thesis was to validate and evaluate the clinical applicability of the latest 3DE technology for quantification of the LV volumes and systolic function with a special attention for LV dyssynchrony determination. Accordingly, the thesis was divided into two parts, namely; quantitative assessment of LV volumes as well as global and regional function using different 3DE modalities (part 1), and quantitative assessment of LV mechanical dyssynchrony (part 2).