Lake Michigan was covered by four synoptic studies (28 and 29 June, also 9 and 10 August) during the summer of 1955. In each study 40 or more complete stations and 50 or more temperature stations were occupied. Data obtained included transparency, temperature, calcium, sodium, silica, wind, sea state, weather, cloud cover, and relative humidity at known height above water. Drift bottles were released at each complete station. Computed parameters included surface currents and probable direction of bottom currents. Both were obtained by the dynamic height method. All the data were synthesized to give tbe best possible delineation of the lake circulation at tbe time of each study. The June cruises were carried out under essentially normal westerly winds and are believed to approximate tbe normal summer circulation. The August cruises took place after winds more easterly than usual and showed an unusual southward current along the east shore of the lake. Winds in tbe Straits of Mackinac apparently exercised a controlling influence over the circulation in a major portion of tbe lake. Primary circulation features in the June studies were tbe outflow current and a large clock-wise eddy against tbe Michigan shore between Grand Haven and Michigan City. The outflow current began near tbe center of tbe western shore, dipped to a variable distance into tbe lower end of tbe lake, reached tbe Michigan shore at Big Sable Point and flowed approximately along this shore to tbe Straits of Mackinac. The large eddy produced a southward current just off the southern part of tbe Michigan shore. The primary feature during the unusual easterly-wind condition studied In August was a wide, strong southward current which flowed along tbe east shore from tbe Straits of Mackinac to the lower end of the lake and then curved into tbe Chicago region. This current displaced to westward (but did not destroy) both tbe outflow current and tbe large clockwise eddy. Two water masses have been recognized in Lake Michigan: Lake Michigan Water and Straits Water. Straits Water was a mixture of Lake Michigan Water and Lake Huron Water and occupied tbe Straits of Mackinac region. Under easterly winds it extended into Lake Michigan as far as Manistique but normally was more closely limited to tbe Straits area. Straits Water is Identical to tbe "Lake Michigan Water" discussed in our studies of Lake Huron (Ayers et al., 1956). Lake Michigan Water occupied tbe major portion of the lake with its northern edge in the island region in tbe north end of the lake. Under easterly winds large volumes of Lake Huron Water were carried into Lake Michigan where they demonstrably diluted the Lake Michigan Water but did not reduce it to the intermediate characteristics of Straits Water. A method of vector plots of wind stress has been developed as a means for studying the relation of wind to current. This method adequately correlated tbe winds at eight stations around Lake Michigan and on the west side of Lake Huron to the currents in the regions adjacent to these stations during seven synoptic surveys in the two lakes. This method indicated that wind stress increments from the ten days prior to a cruise were represented in the lake on the day of a cruise, and that these stress increments underwent a logarithmic decay after being imparted to tbe water.