Conventionally, digital halftoning is accomplished by either changing the size of printed dots or changing the relative density of dots on the page. These two approaches are analogous to amplitude modulation (AM) or frequency modulation (FM) used in communications. A typical AM halftoning method, such as cluster dot screening, has very low computational requirements and good print stability. However, it suffers from low spatial resolution and Moire artifacts. Alternatively, popular FM halftoning methods, such as error diffusion, can achieve high spatial resolution and are free of Moire artifacts but lack the print stability required for electro-photographic printing. In this paper, we present a new class of halftoning algorithms that simultaneously modulate both the size and density of printed dots. We call this new class of algorithms AM/FM halftoning. The major advantages of AM/FM halftoning are: (1). Better stability in shadow area than dispersed dot methods through the formation of larger dot clusters. (2). Better Moire resistance than clustered dot screens through irregular dot placement. (3)The ability to systematically optimize dot size and density to produce the best possible print quality at each gray level. A specific implementation of AM/FM halftoning is developed for use with electro-photographic printers having pulse width modulation (PWM) technology. We present results using dot size and dot density curves obtained through measurement-based optimization, and demonstrate that AM/FM halftoning achieves high spatial resolution, smooth halftone textures, good printing stability, and Moire resistances.