Basic physical principles and concepts plus computer-implemented numberical techniques now make possible a thorough quantitative analysis of acid-base systems. Some important conclusions from that analysis are presented: 1. Acid-base balance for physiological solutions hould be defined as the value of [OH-]/[H+]. 2. pH is a dangerously misleading indirect representation of [H+]. 3. Strong electrolytes affect [H+] and other deendent acid-base variables primarily through their resultant, the strong ion difference. 4 Hydrogen ion concentration in biological solutions is determined by the strong ion difference, the carbon dioxide partial pressure, and the total weak acid present. Changes in hydrogen ion concentration can be broght about only by changing one or more of these three independent variables. The same statements apply to all the other dependent variables, notably bicarbonate ion concentration. None of the dependent variables determines any other dependent variable, although their quantitative behaviors are necessarily correlected. 5. Solutions separated by membranes can interact in acid-base terms only by processes which alter the values of their independent variables. Interaction of intra- and extracellular acid-base balance can only occur by the cell membrane altering these independent variables in the extracellular fluid and in the cytosol.